As someone living in a very touristic place, it's really hard to distinguish Americans from wealthier Brits. You pretty much have to hear them speak. Poorer Brits definitely stand out and I'm guessing so would poorer Americans but we don't get a lot of poor Americans (for obvious reasons). It's easy to spot Scandinavians and as I'm from eastern Europe I can easily pick up Poles and tell them apart from Russians and Ukrainians. Spanish and Italians are also easy to recognize and distinguish. I'm not a waiter so I don't know which ones are the worst customers.