there are various practitioners who are said to have achieved a variety of realizations, up to enlightenment, and some of them are living today. You can't know unless and until you achieve a level of realization where you too are within reach of that goal.
It would be really great if you could see a physical change that was undeniable in every being that achieved the level of arhat or even stream enterer/arya; then we could compare between traditions, and choose the one with the highest percentage, I suppose.
....or even stream enterer