Is it rude to ask if a women is wearing underwear?

Yes, it is very rude. If you are not well enough trained to tell with your eyes, a discreet, well placed hand while escorting your date from the car into the restaurant, or to your table will often let you know for sure. I am not talking about feeling her up or groping around...just a hand placed lightly at the small of her back will tell if she is wearing underwear, and on the center of the back if she is wearing a bra.

Sorry, guys if I have divulged too much information here, but this poor chap needs the help.

Rude Underwear

If you have to ask, then you're probably not in a time or place where the question is appropriate, so yes....it's rude. There are exceptions to every rule, of course: for example if you're paying for a phone sex session, then the question might be entirely acceptable.

Woman, not 'women'.

YES it is rude! Don't do it!