the little things - the way they talk (especially the forms of address) - sometimes the way a couple look at each other or watch each other. Clothing isn't that big a clue any more - a real collar is a dead give-away of course - but leather is everywhere and too many use it as a fashion statement (clothing as well as collars) for it to be a clear sign. There are symbols people use - the real triskele is a good indication - especially if they know what it means - and there are some symbols that have been used (more in Europe) to indicate status (Dom/Domme, slave, available slave) and the like.

And yes - The Look - lol (and sometimes the slight tremble in the sub's knees as she wonders if she ought to drop to her knees right then and there when The Look is meant for her).

But most people are pretty subtle and not obvious , but I'd be willing to bet that most D/s couples in public send out subtle clues all the time. Individuals are harder to identify - but again - sometimes the clues all string together.