You can tell that pi is an irrational because it has a what

I'll guess the answer is you can tell that pi is an irrational because it has a non-terminating yet non-repeating decimal representation.

Of course it's not clear how we tell this. We can't know for sure just by looking at the first trillion digits we've figured out whether it repeats or not. Someone told us it didn't, that's really how we know.