In cities, your best bet for finding a restroom while on the go is to walk into a large hotel as if you're a guest and find the facilities. The next best thing is talking your way into a restaurant bathroom; buying a drink is a nice gesture if you do. If you're staying in a small hotel, don't throw toilet paper into the toilet—use the basket provided—as unsanitary as this may seem. Doing so can clog the antiquated plumbing. Always carry your own supply of tissues or toilet paper, just in case.
Public restrooms are usually designated as "servicios sanitarios" or, sometimes, just "sanitarios".
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The Bathroom Diaries is flush with unsanitized info on restrooms the world over—each one located, reviewed, and rated. www.thebathroomdiaries.com.