I would point out there are multiple such periods and you may need to be more specific. I assume you mean the first Warring States period (5th to 3rd century BC). I'm not very familiar with this period, but historicity is a fascinating subject I can help elaborate one.
Why do we sometimes think a figure is fictional and not a real person? Well, sometimes it's as simple as the person didn't seem to exist.
It can come down to a big maze of literary and physical evidence, comparing things like the apparent importance of figures like with an apparent absence of evidence. Contrary to the saying, absence of evidence is sometimes taken as evidence of absence. It's especially true for big figures like kings or famous warriors, who you'd think would leave ample contemporary evidence of their passage in history.
When historians and archeologists go poking around and find that all references to the great Toltec figure Ce Actl Quetzlcoatl originate centuries after he supposedly lived, they start wondering if that person ever really existed. Coins with their faces. Monuments bearing their names or deeds. Other times we have purported evidence but it doesn't match up with other evidence and we have to go through the muck and find what evidence can be supported by other evidence.
Whether or not Sun Tzu ever really existed has been questioned for centuries (the Chinese were doing it hundreds of years ago). He isn't mentioned in contemporary records of the Spring and Autumn Period and his first historical reference is in The Records of the Grand Historian written centuries later during the Han Dynasty. It's a bizarre omission for a figure who would go on to be so significant. Other far less significant figures are recorded from the period. This challenge has been supported by some arguing that methods of warfare mentioned in The Art of War are not reflective of the time in which Sun Tzu would have lived, but contain later innovations and assumptions.
Thus some propose, Sun Tzu didn't actually exist or isn't the person we thought he was.
Sometimes it get murkier.
The figure of Diao Chan (a figure from the 3 Kingdoms Era) is fictional. She comes from a storytelling tradition that rose after the period. She has some vague basis in history however. The Records of the Three Kingdoms, a historical source that is distinct from the dramatic Romance of the Three Kingdoms, references Lu Bu fearing that Dong Zhuo would discover an affair he was having with one of his maids. The maid is not named or mentioned ever again and it appears in the record as an uncorroborated rumor.
But the rumor became the basis for the figure that became Diao Chan.
So really there's multiple levels here. There's historical figure some argue never existed, other figures who maybe existed but who's name and memory is largely fictional, and then other figures like who existed but we have to wonder and question how much of the information we have for them is reliable.
The Warring States is the period leading up to Qin's unification of China, if your asking about the three kingdoms in future (while strictly speaking 220-284, the civil war leading up to it ie from Dong Zhuo seizing power is generally accepted) then use the term three kingdoms. It makes it far easier to know which era you are talking about
I did work on Kongming.net Encyclopaedia (sadly the site is down) that tried to list every officer of the three kingdoms including if they were historical or fictional. As has been mentioned, we have the records of the three kingdoms, taken from each kingdom's library and records (Wei and Wu had their own history projects, Shu had their record departments which Chen Shou served in but never had their own history project which was a key problem) and compiled by Chen Shou, an officer of Shu and Jin. It's generally considered a remarkably neutral work while Pei Songzhi of the Liu Song dynasty later added annotations to flesh it out, which were either other historical works about that time or commentary from figures like Sun Sheng as well as his own
It is unlikely any of the three kingdom record departments added anybody fictional. Some figures only get mentioned once, in some cases only in a hostile memorial from one side attacking the other for wrong-doing, others there can be confusion in the texts where it is possible somebody is one figure with different names or separate figures. Or a figure turns up once and there are contradictory accounts about what happened. The annotations include contemporary accounts like the work of Jing and Wei scholar Wang Can, others are of later works like Chang Qu's local history that tells us of Meng Huo.
How reliable are they? Depends on the source. Some are usually reliable, others have a clear bias be it for a kingdom or a hatchet job against another, others can be met with disdain by Pei Songzhi and later historians for their truthfulness. There are some fantastical tales within the texts, particularly where mystics are concerned but not only them.
I can't recall a strong accusation that any of the works invented a person though bar ones of folktale (Zhong Yao's ghost girlfriend is probably not real). Meng Huo there have been questions over given the name meaning but he is included in works from Sima Guang's ZZTJ, Rafe De Crespigny's Generals of the South, Bin Yang's Between Wind and Clouds or John Herman's "The Kingdoms of Nanzhong China's Southwest Border Region Prior to the Eighth Century". The name does raise an eyebrow and somewhere there could be an exaggeration or name play by Chang Qu but names had multiple meanings, Liu Shan's name was used by some in Shu like Qiao Zhou to predict Shu's fall.
Over time, Shu became popular, Liu Bei and his brothers fighting against the larger power led by the craftier Cao Cao, sometimes assisted by the sage adviser Zhuge Liang, the peace of the Han held an appeal during times of civil war and short-lived dynasties. Tales, as mentioned, like Guan Yu's journey became plays, figures like Guan Suo and Zhou Cang became invented for plays and other fictional works.
Then we have, several centuries later, the famed romance of the three kingdoms that tapped into existing plays and tales and build its own narrative. Building on tales that come before it is pro-Shu (though the backlash against Shu doe that can cause issues), Liu Bei (though his abilities get diminished) is made into a figure of virtue and close to a morally ideal ruler, Zhuge Liang goes from genius and capable commander to the smartest guy in the entire land which only heaven, his own side and the not quite as smart Sima Yi can stop. It borrows from the works that came before it, figures like Guan Suo (a brief cameo) and Zhou Cang have roles in it.
The novel also fundamentally changes a lot of things (power dynamics, who was loyal to the Han, so on and so forth, in one case turning a minister into a eunuch) including the way battles were fought. An era of few duels (Lu Bu vs Guo Si, Taishi Ci vs Sun Ce, Yan Xing vs Ma Chao) turns into duels, duels and more duels. An era of mighty warriors whose displays of combat could turn a war by beating other champions and so a lot of officer on officer killings. Some of those killed are historical figures who the novel feels is expendable or who it builds up so the kill on that figure is impressive but there is also a lot of fictional figures to send into the meat grinder.
Sometimes when the novel drastically changes history or has to create a campaign out of very details (or re-hauls it), fictional figures come in. Ma Teng needs to die a Han loyalist so a fictional plot with fictional characters comes in, which also allows various other messages to be played up, to set out how Ma Teng dies. Or Zhuge Liang vs the Nanman is turned from, though important for resources and stability, a short campaign against rebellious local magnates into a multi-chapter epic where Zhuge Liang is the civilising scholar from China vs barbarians who need to be taught the filial and correct ways. A lot of fictional officers are added for this grand campaign full of poisons swamps, a woman fighting, elephants and all sorts.
If a figure is only in the novel or other fictional works (like the old plays or modern adaptions) then it is an author's invention and not history. We can't go, in the case of Guan Suo, "well Guan Yu had a third child that the records, for no discernible reason, simply forgot to tell us about and wors of intended fiction just happened to tell us the truth". If not in any historical record but simply in works of fiction, we have to assume it is fiction.
If you are new to the era and trying to work out who is fictional? That is trickier. Figures with magical powers (Zuo Ci for example) in the novel might be the first one people guess as novel fantasy at but are often in the records and mystics were a recognized part of the era. The Nanman chapters contain a few figures that were historical (if perhaps not actually Nanman) amidst the many fictional figures. There are many fictional fodder generals but some of the seemingly fodder generals are also historical (Qu Yi is reduced from Yuan Shao's main commander to an early kill for Zhao Yun and to make way for Yan Liang and Wen Chou for example), plenty like Zhao Cen or Guan Suo are simply there and not obvious potential candidates to be fictional.
Bar asking (if you have a particular figure in mind that you are wondering if real or not, do drop me a line), the only way is learning enough of the history rather then the fictional version of the era and knowing who isn't in it.