A few ways to approach this: Look for the bathrom - sign will say "Toilette" in your restaurant, hotel, or museum before you leave. If you are out in the streets of Rome - I strongly suggest you look for the largest hotel - walk straight through the lobby to the bar and ask directions (they will speak English). The bathroom in these hotels are elegant and accessible. If you go into a bar, you're expected to make a token purchase if you use the bathroom - I don't recommend it in the ordinary bar - you are apt to find no toilette paper and no toilet seat (you wouldn't want to sit if there were one available) - always carry extra Kleenex with you for such emergencies. Often men and women use the same facility. Avoid McDonald's - always nasty and no paper. Vatican is immaculate and someone is there cleaning constantly. Always carry a nice little supply of paper napkins with you as even nice restaurants seem to neglect this little necessity and can have no toilet paper.
Again - the large hotels are your best bet - dash off the streets to heed Mother nature's call.
If you're really lucky - you be near Piazza Barbarini and can head downstairs to the lovely facilities in the Hotel Bernini. Photos here to guide you along - please don't tell them Carol sent you!:)

