If you were in a public place in the US, you would ask "Where's the men's room/ladies' room or "where are the restrooms?" note: Ladies' Room, not Lady's Room.

US - I would use "restroom," "men's room," or "ladies' room" in a public space (like a store or museum or restaurant) "bathroom" in a private space (someone's home). "Washroom" is very Canadian. "WC" is very British. There are other terms. Many are less polite.

Below they seem to answer your question, about how to refer to thre room. Perhaps, however, even more common in some areas and with some people would be totally different expressions. "little boy's room" <-- humourous "Jon", John? Where's the toilet? Where are the toilets? Head? conveniences Sorry, I'm not much for slang, but perhaps others can give you some of the dozens of slang terms.

It's mostly common to say "restroom" and "bathroom." Between the terms, there are no differences. Some are gender specific but besides that there are no differences.

people say this, or you can say bathroom or restroom also.