Or... if you feel you must, modify your profile so that you tagline (that appears directly under your name,) says "Today I'm Topping" or "Bottom Beggar" or something similar.

That said, it still requires someone interested to go look.

The easiest way, as denuseri points out, is to tell people as the beging conversing with you.

Of course, please try to avoid referring to yourself as The Great and Powerful Oz... as that would be me.