I just do the polite thing and go about with business as usual. If they happen to become a part of business as usual, I act as I'd act with anyone else.
Except for an accidental event a few weeks ago... as a little game, one of my friends and I like to pick people in the mall to stalk. We make up little stories, try to figure out which shops they're going to go into, and follow them until they go into one (then we pick someone else). Call it active peoplewatching.

And we're not creepy about it; we stick way back and just talk as if we're talking about anything else.
Well, a couple weeks ago, I see this woman with a haircut that I instantly fell in love with. It was short and spiky, and I bemoaned to my friend that I wished I could do my hair like that and still be seen as totally feminine. So we began to follow her. She walked all the way from one end of the mall to the other, went up the escalator, back to the end where we started the chase, then to the food court in the center of the mall. Her walk was brisk and purposeful, and her head never looked sideways into any of the shops. Well, when she got to the food court, she turned so we could see her side profile, waved to this guy in the center who then walked over to her, they hugged, and walked out of the mall hand-in-hand. And it wasn't until she turned to the side that I noticed her adam's apple and browbone... she was definitely trans, and I buried my head into my friends shoulder to stop myself from busting out laughing about what we had just done.
As I told her at the time, I,
of all people, should not have been so surprised to see that.