Compare your outward appearance and behaviors to those of people who live in the country you visit. The more you blend in, the less chance there is for you to look like a tourist. Some simple ways to spot a US citizen in a crowd overseas include the following. I spot American tourists mostly because of their skin complexion. A lot of them tend to be whiter or darker than the Spaniards.
Wearing baseball or trucker caps Bah. Europeans wear it too, and I have seen french and spaniards wearing New York Yankees baseball caps.
Wearing US brand name clothing with the names (ie. Nike, Gap, Abercrombie, Patriotic, Texas A&M, etc.) easily readable Bah. I've seen many locals wearing those brands everywhere. The rest of the world also travels!
Wearing white socks with shoes instead of dark socks. Well, if you do that in Spain you will not be considered a tourist, just tacky.
Wearing shirts tucked in or left hanging out, depending on the country you are visiting. Sometimes this differs for men and women in the same country. In some countries, NOT wearing something could signal that you are a tourist. For example, in some countries, hats or scarves are worn by the majority, or, in winter, most people might wear neck scarves. Be alert to simple, inexpensive ways that you could blend in. This is stupid.
Avoid walking down the street while eating food. Requesting, at a restaurant, typical US condiments like catsup, salt, pepper, etc. It is best to use the typical condiments in your host country, but if you cannot eat without some condiment, take small packets with you to use rather than insisting that the restaurant supply you with it. Don't insist on drinking “Coke” with every meal. Drink whatever you want.
Try to manage without requesting ice in your drinks. This one is probably the only one I could agree with
Remember that often “bottled water” will be carbonated. Before the waiter brings it and opens it, be sure to request water that is “natural” instead of “with gas”, if that is what you prefer. Use local table manners instead of US ones. For example, in Europe, a fork and knife are used differently than in the US. In some Southeast Asian nations chopsticks might be more appropriate. And how a newly arrived US citizen is supposed to know that?
Don't insist on ordering meals in a typical US fashion. For example, in some countries, a salad is the last item served, not the first. In others, people don't eat what we in the US think of as a “salad”. Be conscious of your behavior in public places. Here are some examples that could signal you are a US citizen: Avoid making eye contact, smiling, greeting, and engaging in conversation with strangers. Spaniards are experts in smiling and doing eye contact with strangers.
Refrain from carrying US newspapers or books in plain view. Why?
Refrain from loud talking, large arm and hand movements, and boisterous behavior. Adults in most cultures are more reserved and use lower voices in public places (i.e., train stations, shopping centers, public streets, etc.). If you do that in Spain you'll look like a local!
If you must use a dictionary to translate a sign or menu, be discrete. For example, copy down the words of the sign and move aside to a less public place to work out the translation.
Why?
Avoid showing other people what you have purchased by pulling items out of shopping bags in public places such as streets, busses, etc. Spaniards do it all the time.
Be conscious of the amount of space you occupy; most cultures are used to using less than US citizens. For example, try not to occupy more space than locals at a restaurant counter, in a waiting area in the train station, in the bus, etc. Avoid chewing gum in public places. This is stupid.