My college roommate and later lead singer for my first "real" band (as in playing paid gigs and everything) could speak in tongues, and after all the time we'd spent together, everything we'd been through together, I believe it. He ended up leaving the band because he felt that he'd been called into God's service and could no longer participate in what we were doing.
Before he left, we had a number of talks. From a selfish standpoint, I didn't want to lose my best friend and lead singer, but I also wanted to be sure he knew what he was doing. He said he was sure, absolutely sure. He'd been called, and he had to answer. I asked him how he knew, why was he so sure.
He said, "Bob, I can speak in tongues. I'm not making this up. I have no reason to make this up. I don't want to quit rock and roll, I don't want to leave all you guys, but this is serious." Then he bowed his head for a moment, and when he looked up he had this odd look on his face, and spoke a few sentences. The voice that came out of his mouth was not his, and it didn't sound like gibberish to me, but it sounded like an ancient voice speaking in an ancient language. It was like nothing I'd ever seen or heard before or since, and I know he wasn't faking it. There's just no way he could fool me.
He blinked a few times, then looked at me and said "Did it work?"
"Did what work?" I asked.
"Did I speak in tongues, just now? I never know when it's gonna happen, and I can't really control it, not yet. But it's important to me that you believe me, and understand what's happening here. So I asked Him for a little help, so you would believe me."
I believe he did speak in tongues that night, from that event and from the other conversations we had, and from knowing the guy inside and out for years. He's been a fully ordained minister for over 20 years now. He's the real deal.