@Dina496 Hosts in various places where licenses are required will usually state so in their listing desciption, and list their license number. But you'd actually have to look at each listing to determine that, as far as I'm aware there isn't any filter you can use for it.
No,I don't think it's rude to ask, it's a legitimate concern. It would only come across as rude if you just solely asked "Do you have a license to operate?" which sounds like some accusation, or even could make a host wary that you are some kind of govt. inspector. Just ask politely, stating the concern you mentioned, while at the same time volunteering some info about yourself and asking any other questions you might have about the listing.