I won't say that there aren't those out there who posses something unique or different. There are, of course, always exceptions to the rules. However, there are steps you can take to determine one's credibility.
1) Is there any background information that connects what the person says to be to what they say out loud or in public places like the forum. I.E. if their bio is one filled with a long narrative about their life struggling with being an alien.. you can assume it is false. If, by chance, they were an alien, they would keep this to themselves and probably wouldnt be on this site.
2) Is the person easily offended by the topic of what they claim to be? For instance, most role players who claim to be demons will not know anything about the true demonology and will post many things either hinting at what they are or arguing against others about the subject. Their ignorance tends to be outlined by a thick cloud of what I call'internet-anger'.
3) Is there any proof as to what the person claims? Sometimes seeing isnt believing- but if you aren't sure, this is the most obvious step. If the person claims to be something they should be able to back up the claim.
MerMAIDgirrrl: Oh my gawd u guys im a meraid!!!!1
SensibleHumanMale: Are you really?
MerMAIDgirrrl splashes watr @ u angry like
MerMAIDgirrrl: im totes mermaid i have a mertail and flippers lol
SensibleHumanMale: Can I see a picture of your tail?
MerMAIDgirrrl: NOe WAY! STALKER! I cant show stuff like that ill get caught n taken to a sciene lab!
SensibleHumanMale: Your grammar and punctuation are atrocious. Though I can see that typing with fins must be difficult. Is your computer waterproof?
MerMAIDgirrrl leaves chat