We worry about making a comment to someone we think may be a crossdresser when we see them out shopping. We don't want to out them and make them feel their presentation is poor and we don't want to discover we complimented a FAB for looking like a woman, ouch!
It seems the answer is to just be natural and if you see an outfit, or jewelry or shoes, whatever, that you like, just make the compliment. Most of the time it will be women who are on the receiving end and if you are honest and non-threatening, they should appreciate that their efforts have been recognized and appreciated. If you make the comment and the person is a crossdresser it should improve their day as well. If you have been making compliments on a regular basis it should come across as just that, a compliment. Then they may decide you are not a threat or a jerk, and there could be more conversation to follow. If not, you made someone smile.
We practice doing our makeup, picking out outfits and perfecting our presentation. Why not practice being appreciative of our inspiration?