You can tell it's horseshit because if they were actually performing the miracle of Speaking in Tongues that they claim to be you would be able to understand whatever they're babbling about regardless of what language you speak. If they were genuinely doing it you probably shouldn't even be able to tell unless someone who speaks a completely different language comes along and says something like, "Wow, it's crazy that this congregation prays in Swahili," or whatever.
It must be reassuring to see the world in simple absolutes.
Jesus sure did:
Whoever believes in him is not condemned, but whoever does not believe stands condemned already because they have not believed in the name of God’s one and only Son.
Most things aren't absolute, but being able to recognize that some things are absolutely nuts and worth ridiculing because of the amount of harm and suffering they cause, is kind of reassuring.
I grew up with two parents who are always engaging in glossolalia. They'll swear black and blue it's a language that connects them with god. To them it's not complete horseshit. But it is.