In the United States, the more polite terms I hear are usually either the restroom or the ladies' room/men's room (always including room). Washroom sounds Canadian to me. British terminology (such as loo) may well be understood but would sound odd.
So, I might ask: "Where's the restroom?" If I already knew, I'd most likely excuse myself without explanation.
BEWARE: In the United States, using the word toilet to mean anything other than the actual porcelain fixture sounds shocking or rude. The difference between British and American English in this regard is quite pronounced. The first time I visited the UK, I was quite surprised to see signs which read "Toilet." You would never see such in the US.
Also, I've encountered people in the UK who don't understand the word restroom, which is usual in the US. So, even within English, there's significant variation from region to region.
P.S.: In the US, bathroom and restroom are synonyms. However, bathroom is more informal, and is more likely to be used in a house than in a public building. Restroom, while more formal, is quite acceptable in all situations and is the best default choice if you're unsure which is the best option. That said, using bathroom in a formal situation would only be a minor gaffe.