How did James Bond know that Peaceful Fountains of Desire holstered a pistol on her leg, before pulling up her dress?

He knew that the hotel is a front for Chinese intelligence. He knew the staff are all spies. She had to have a concealed weapon, and there's a limited number of places where she could hide it.

I do not quite understand this question. It has been years since I saw this movie. but, IIRC, the two characters involved are in a location and situation where pulling up someone’s dress would be rather aggressive and presumptuous. But, not out of the ordinary.

Also, she might not have considered it strange considering her name. Just an occupational hazard. And, Bond is Bond. He would have just proceeded to do what Bond normally does to pretty women. And, spies are somewhat outside of the law anyway.

if we assume that Bond is actively searching for something out of place, he could sense the thickness of the weapon holding belt on her tight. Of course, it could have been for example a garter, but since her legs are bare (and she is not a stripper), it is most likely something more sinister than a piece of lingerie.