Yes just join a level that you think you are and see how you do. I would play at least 2-3 tournaments at the same level before you decide that you are not at that level-unless you are winning very easily or losing very badly for every match. I know one guy who played juniors and turned 19. He didn't know what to rate so he played opens. He got smoked 3 tourneys in a row so he dropped to 3.5. He won two tournaments in a row and some 3.5 regulars were complaining pretty loudly to the tourney director that he was sandbagging. He then moved to 4.0 and kind of settled there.
At the end of the year you will get a rating or at least that is what happened to me.