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.
When asking questions ALWAYS post the name and a link for the pattern if you have it.
Sometimes its arbitrary. Just pick one if one side looks like another. If that's the case, I put a marker on the right side so I know which is which. Its always the side that everyone else will see when you wear it.