At first glance, I would probably look the typical teenager; baggy jeans, and long sleeved shirt (though I do live in the tropics, so that is probably odd). If you look closer though, you may notice the miraculous medal, and the crucifix, and brown scapular (I have them all on one chain around my neck). And perhaps you may notice that I may half-way pull out a rosary out of my pocket every now and then.
Not saying this to brag, I"m just saying to look for the signs. Perhaps you will notice that this said traditionalist will bow his head before eating in the cafeteria and mutter under his breath. Perhaps he will have a twist in his mouth when he sees an immodest girl or woman, or look away quickly. If you are in high school, they will probably hang out with the nerds, or the more reserved people (especially if it is public, since they are the only ones that will be anywhere near modest). Just think of what a catholic should be like, and how they would react in a very wordly world. There is always the chance that they are part of a different religion, but it doesn't hurt to ask if they act Catholic. Though, Even if they were Catholic, but wearing skimpy shorts, I probably wouldn't rely on them for religious support, if that is what you are looking for.
I guess there's the obvious too. There are Trad Catholic slogans out there. My favorite one I believe is this guy dressed up as a priest from the 20's with a pipe in his mouth. There could also be the medals, or the other devotions to the saints around that person's neck, that could mark him Trad Catholic.