Depends on what cast on you used. Long tail has the tail on the same side as the working yarn. Cable cast on and knitted cast on it's on the other end. Not sure off hand about other methods so you'd have to check.
I find that if I need to know which side is which I'll put a safety pin to mark the front side. Stockinette is easy, but something like garter you'd need to mark if it's important.