He looks her up and down. Since she's displayed herself, however subtly, it would only be polite. It would also, however, be courteous to be equally as subtle about his appreciation, so the only sign is a slight widening of the smile and lift of the eyebrow.

"I've found that that's a judgment usually to be made upon meeting the lady in question, rather than a set of criteria blindly applied." Truth, really. Even if it means little by itself.