I know the wrong side is the side not shown on a piece but which side is the cast-on tail? If it is on the left does that indicate the wrong side is facing up?
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.
Depending on the edge treatment and the design the tail from the cast on (regardless of what cast on is used) could be on either side. It depends on the design.
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.