Demonstrating the legitimacy of modern tongues would be a perfectly simple matter. I Corinthians is very straightforward: “If anyone speaks in a tongue, let there be two or at the most three, each in turn, and let one interpret. But if there is no interpreter, let him keep silent in church, and let him speak to himself and to God.” The speaking is not meant to be mere noise — it is real speaking, meant to convey information, as passed along by the interpreter.
So you record the alleged tongue-speaking then play it back for several alleged interpreters. If different interpreters all give different translations, then at least all but one of them are fraudulent. If an interpreter gives a different translation for the same recording at different times, he is fraudulent.
If there are no legitimate interpreters, then the alleged tongue-speaker is, at best, just making noise and ought to keep it to himself.
That's my understanding of the issue.
The thing about speaking in tongues is that anyone can do it if they enter into an ecstatic state. You don’t have to be a Christian or to even believe in God. The hard part is to decipher the source of the behavior.
"I thank God that I speak in tongues more than all of you. Nevertheless, in church I would rather speak five words with my mind in order to instruct others, than ten thousand words in a tongue."
Paul points out the differences in using tongues in private vs. public settings.
In private he speaks in tongues more than anyone presumably for self-edification.
But in church the tongues should be interpreted for the benefit of others.