My friend Calle drew my attention to the fact that the well-renowed linguist Lyle Campbell has written an article on how to spot that a speaker is actually faking knowledge about a language. Interesting, huh? And, you can read this article for free online!
Link to free online PDF of official version
Link to in press-academia version (in case the above one has problems)
Why would a person try to fabricate a language, why one would engage in such deception? Whatever the full range of motives might be, two significant ones almost certainly involve money and status.
As mentioned above, fakers run out of steam after a short while, not able to continue to come up with new “words.”
The faked languages I have encountered exhibit none of the recurrent parts of “words” that might be associated with inflectional or derivational morphology in true languages. Fakers seem incapable of fabricating morphology.
In short, it turns out to be very easy to detect attempts to fake a language in these situations – they exhibit the characteristics pointed out here.