The old saws that "you can't tell a book by its cover" or "you can't tell a dude by his duds" have only a certain degree of truth to them. Both are warnings against hasty decision making.
On the other hand, with the availability of t-shirts with slogans on them, you might make headway some of the time. Still, there's the simile of a trout struggling upstream that comes to mind.
But is there any easy-peasy way of judging Trump supporters?
If you care, it's in their hair. In a limited way.
Specifically, if he's sporting a mullet, he's likely to be a Trump supporter. Or, she's got big hair, she's likely to be a Trumpette.
Of course, there are the Trump supporters who pass for normal by getting mainstream 'dos. But mullets are so Trumpian; maybe Rudy Giuliani should get one.