For the time, this was quite an easy feat and many ships could accomplish it. Starting in the late 1700's, merchant ships frequently carried small complements of cannon in order to protect against raiders and pirates. These complements could range from the small 4-6 pound guns seen on smaller brigs to the capable 12-24 pound cannons and carronades seen on the many East Indiamen that traded between Asia and Europe. That said, a fair amount of merchant ships carried no cannons to make room for more cargo, but either still had gun ports to accommodate cannons if they were needed, or would actually paint on "fake" gun ports to create the illusion of a heavily armed ship to deter pirates. This meant that the process of identifying who was an unarmed trader, a heavily armed merchant, and vessel of the King's (or Queen's) Navy was actually quite a problem for those who engaged in raiding as demonstrated below.

On the 21st of December, 1800, the HMS Speedy under the command of then Commander Thomas Cochrane described an incident where he had spotted what he believed to be an unarmed merchant vessel near Plane Island. The Speedy then proceeded to close with the vessel, expecting an easy capture, when suddenly the ship threw open its gun ports and filled the deck with sailors that had been previously hidden below, declaring itself as a Spanish frigate. The only thing that saved the Speedy was the fact that Cochrane had actually repainted the ship a few days earlier to look Dutch and had hired a Dutch quartermaster who told the Spanish that the ship was infected with the plague. This scared off the Spanish and they let the Speedy pass, but for our purposes, we can note how easy it was for a fairly experienced commander not be able to distinguish between a merchant and an enemy vessel, or conversely, how easy it is for a warship to disguise itself as a merchant.

Now, this problem of distinguishing and disguising vessels was even worse for the famed East Indiaman vessels. These were large cargo ships that were capable of carrying large armaments that could match frigates and even ships of the line, as a handful of East Indiamen were bought and used by the Royal Navy as fourth-rate vessels, an example of this being the Warley which would later become the HMS Calcutta. In his book The East Indiamen, author Russell Miller writes:

The lines and rigging of a 1,200-ton East Indiaman virtually matched those of a 74-gun ship, and an 800-ton vessel could easily pass for a frigate. The East India Company made the most of this happy resemblance. Acting like pugnacious Royal Navy Captains, [East India Company officers] steered staight towards the enemy - an intimidating tactic that also concealed an Indiaman's feeble broadside. (154)

In January 1797, outside of Canton, French Rear Admiral Marquis de Sercey spotted a grouping of six British ships and dispatched one of his frigates, the Cybele, to reconnoiter the fleet. As the Cybele got closer to the vessels, they suddenly hoisted the ensigns of the Royal Navy with one of the lead ships raising the blue of a Rear Admiral. Two of the British ships broke off and began to pursue Cybele, which promptly turned and ran back to rejoin Sercey's main force. They signaled that the force was comprised of four frigates and two ships of the line under the command of British Rear Admiral Peter Rainier of the East India Squadron. Sercey quickly withdrew his six frigates, fearing himself outgunned and outmatched, moving so quickly that one of his frigates snapped her main-topmast after overcrowding her sails. Only later did Sercey discover that not only was the East India Squadron and Peter Rainier nowhere near Canton, but that the four frigates and two ships of the line were actually just six half-manned and half-armed East Indiamen.

An even more humourous example than the last is a small action which took place on August 14th of 1800. Off the coast of Brazil, three French frigates met with a ten ship convoy consisting of one 64-gun HMS Belliqueux, four 30-gun East Indiamen, and a handful of lightly armed merchant ships. The French initially closed with the spread-out convoy, hoping to bag one of the merchant ships, but began to flee upon seeing the two-tiered gun decks of the Indiamen. Belliqueux broke off to pursue the French flagship while ordering the four Indiamen to pursue the other two frigates. By midnight, the pursuit had been split and the British 30-gun Exeter was slowly but surely closing on the French 36-gun Medee. However, the Exeter only had a crew of 130 compared to the 315 serving on the Medee and Captain Meriton of the Exeter was concerned that if the Medee realized this and turned to engage, the Exeter would have no chance of survival. So to intimidate the French, he had his crew place lanterns behind all of the Exeter's gun ports to light the ship like a "fearsome, leering, jack-o-lantern." He then had the ship run up alongside the Medee and "in stentorian tones," demanded the surrender of the vessel. Once again from The East Indiamen:

The French captain, imagining himself under the guns of a British warship, instantly complied, hauling down his flag. As he climbed onto the Exeter's deck to surrender his sword, the frenchman stared in disbelief at the tiny 18-pound carronades there and asked what ship he had surrendered to. "To a merchantman." Meriton replied. The chagrined Frenchman, seeking to redeem his honor, pleaded to return with his crew to the Medee and proceed with a proper battle - a request that Meriton, not unreasonably, declined. (155)

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Citations:

Cochrane, Thomas. The Autobiography of a Seaman. 1860. https://www.google.com/books/edition/The_Autobiography_of_a_Seaman/E9tCAAAAIAAJ?hl=en&gbpv=0

James, William. The Naval History of Great Britain, from the Declaration of War by France in 1793, to the Accession of George IV. 1847. https://archive.org/details/bub_gb_Yc09AAAAcAAJ/

Clowes, William. The Royal Navy: A History From The Earliest Times to 1900 Vol.IV. 1898. https://archive.org/details/royalnavyhistory00clow_392/

Miller, Russell. The East Indiamen. January 1st, 1980. https://archive.org/details/isbn_0809426897/page/154/mode/2up