This is a great question!

Perhaps the two most famous French novels set in the decades immediately following Napoleon's rule both involve noble titles of questionable provenance as plot points: In Les Misérables, Marius' father was made a baron by Napoleon on the field of Waterloo, but the next regime refused to recognize the title; in The Count of Monte Cristo both the titular character and his enemy Fernand Mondego use wealth to fabricate noble titles.

Both examples highlight the intricacies of your question. Some legitimate titles were questioned; some people did fraudulently claim nobility. Let's take a look at the situation of noble titles in early 19th Century France, and where the "new nobility" created by Napoleon fit in.

First, an extremely quick background: the ancien régime before the Revolution of course had a class of titled, hereditary aristocrats. The old regime nobles had a vast array of gradations, but are (over-simplistically) divided into two groups: the noblesse d'épée or Nobles of the Sword, who dated their titles back to the medieval military aristocracy, and the noblesse de robe or Nobles of the Robe, who were given their titles more recently in response for holding (or purchasing) various official government offices. When the French Revolution came along, all noble titles and privileges were abolished and all citizens declared equal before the law. It wasn't illegal to have come from a noble family, but there were actual laws passed criminalizing the display of feudal coats of arms.

After Napoleon declared himself emperor, he eventually started bestowing his own titles of nobility. Napoleon never recognized the ancien régime noble titles, and around 22 percent of the people he granted titles to had previously held noble titles before 1789. But the lion's share of Napoleon's titles were given out to soldiers, officerholders, and members of social groups that had risen in prominence since the Revolution.

In 1814, when Napoleon abdicated and the old Bourbon royals returned to rule France, King Louis XVIII issued a constitution for France, the Charter of 1814, whose Article 71 stated, "The old nobility resume their titles. The new retain theirs. The king makes nobles at will, but he grants to them only ranks and honors, without any exemption from the burdens and duties of society." This was a compromise measure aimed at healing a split in French society between the old and new orders, or at least an attempt to not alienate a significant portion of 1814 France's elite.

The second sentence of that quote from the 1814 Charter is important too, though — "The king makes nobles at will." Louis XVIII, and later kings Charles X and Louis-Philippe, both bestowed titles of nobility at will.

All told, Napoleon gave out around 3,000 noble titles; the Bourbon Restoration would grant around 4,000 new titles, more than two-thirds of them in its first few years. Just as 22 percent of Napoleon's entitlements went to people who already had ancien régime titles, so 27 percent of people receiving Bourbon titles after 1814 had previously had imperial titles. So you can see how things could get confusing! You have multiple regimes granting noble titles over a period of decades, with some people holding titles from multiple different regimes. For example, Armand-Augustin-Louis de Caulaincourt, a close advisor to Napoleon, came from an Old Regime family from whom he inherited the title "Marquis de Caulaincourt." Napoleon made him "Duc de Vicence." Which title these double-nobles used tended to depend on the regime in power.

(I will note, relevant to the plot of Les Misérables, that during his final days in power before abdicating, Napoleon entitled just 12 comtes and five barons.)

The key point in all of this, though, is that there was no centralized record of noble titles. Both Louis XIV and Louis XV conducted "registrations" of nobles, but this was done for the purpose of taxing them and are not seen as reliable or complete. The closest thing to an official record in the Old Regime was a family of hereditary genealogists, the Hoziers, who over several centuries passed down the role of juge d'armes de France from father to son.* If there was a question about the validity of someone's noble claim, the current Hozier would conduct an investigation and render a report. The Hoziers also published formal genealogies of the French nobility, including a revised edition that came out in 1823.

But the last Hozier, with his semi-official status as genealogist for the king, was by this time competing with a host of unofficial volumes, often with a sort of pay-to-play style — one 1817 prospectus tried to get nobles to subscribe to the forthcoming volume with the inducement that if they did subscribe, they could guarantee that the volume would print their name, title and residence. This business of producing noble genealogies could be fierce; we have plenty of records of various genealogists accusing each other of being frauds. If you were chronicling the nobility, it helped a lot if you were noble yourself — "among people as touchy as nobles," historian David Higgs writes, "the credibility of a nobiliaire was crucial. To be listed in the right one was heaven; to appear in the pages of one thought to be the work of an unscrupulous compiler was ridiculous." In 1827, the comte de Croy-Chanel denounced one such genealogist, the so-called Chevalier de Courcelles, accusing him in an open letter of having no claim to the title of "chevalier" and "being in fact named Jullien, a former notary at Orléans." In 1845 one genealogist attacked a rival in the street.

(It's worth noting that this development was not at all exclusive to France. The year 1826 saw the publication in England of the first edition of Burke's Peerage, a compilation of British nobility.)

So with no formal or centralized way to be certain of someone's noble status, people often noble status to which they had no legal right. Now, actually claiming to be, say, a count, was somewhat rarer than just adopting the particle "de" to your name, which carried connotations of nobility. Thus the writer Honoré Balzac rebranded himself as "Honoré de Balzac"; the father of the writer Guy de Maupassant dug back in the family genealogy to discover a noble ancestor, thus justifying his use of the particle. In the French parliaments 1814 to 1834, anywhere from 20 percent to 35 percent of elected deputies were classified by a later historian as "near noble" status, which includes people whose use of the particle appears doubtful (as well as others who were on a path to become noble until the Revolution interrupted things).

So all that takes us back to the original question: were Napoleonic titles more suspicious?

If anything, the opposite is true. If you (or your father) had been given a title by Napoleon, you probably still had the paperwork backing this up. The same might not be said for someone who had been ennobled by Henri IV in the 16th Century. As historian Guillaume de Bertier de Sauvigny notes, "The old aristocracy held very few titles which had been given by royal warrant or letters patent in the ordinary way."

So one 1817 pamphlet lamented, "By contrast with the former nobility, one had the new, wherein all was constant and certain." As Higgs notes, "It was a novel thought that recent titles were less open to question than the old." That doesn't just apply to the imperial nobility, of course; those ennobled under the Restoration similarly had the benefit of recency.

As a result, some old-line nobles under the Restoration gathered up the evidence they could find and tried to get new letters patent from the king, belatedly certifying their nobility in case anyone challenged them.

This is not to say that it was better to be newly entitled. From the perspective of old-line aristocrats, the more ancient your lineage, the better. There were great social feuds and examples of snobbery between different groups of nobility, with old-line nobility disdaining the parvenu Napoleonic nobility, who often returned the scorn.

As Monte Cristo tells Baron Danglars about the allegedly blue-blooded Comte de Montcerf, "your name is popular, and does honor to the title they have bestowed upon you; but you are too intelligent not to know that according to a prejudice too firmly rooted to be exterminated, a nobility which dates back five centuries is worth more than one that can reckon only twenty years."

As it happens, though, Fernand Mondego's identity as the Comte de Montcerf was a complete fabrication — but one that he's able to pull off for many years without any serious issues until Monte Cristo arrives on the scene. The prospect of having this identity exposed is seen as a major social disaster — money alone was not enough to buy one's station. The more realistic Danglars, who was of equally low birth but received his title legally (by virtue of being rich), retorts to Monte Cristo that "though I am not a baron by birth, my real name is at least Danglars... I have been made a baron, so that I actually am one; he made himself a count, so that he is not one at all."

In Les Misérables, Georges Pontmercy, whose battlefield title of baron was not recognized under the Restoration, would have been something of a unique case. The Bourbons recognized Napoleonic titles of nobility granted before 1814, and only drew the line at the handful of titles granted during the chaos of 1815. Pontmercy perhaps had the bad luck to die circa 1827; after France's 1830 Revolution brought in a change of regime, figures like Marshal Grouchy, also ennobled during the Hundred Days, were restored to the Chamber of Peers.