From the Paris thread. I am by no means ashamed to be an American, but I certainly don’t want to be a target.
When you are in Europe, what screams American to you?
I have no idea. My husband and I spent a year in Germany in the 1980s, and everyone seemed to know we were Americans (they would speak English to us right away), even though Germany and Germans were the most American-seeming places and people of any I’ve encountered not in the United States.
I’m in Rome right now. You can not tell who is what nationality. I’m serious. I mean the Japanese look Japanese lol but Europeans look like Americans and Americans look like Europeans.
Everyone wears Nikes and converse. The north face is popular. The only way I could tell you that someone was American is when they’ve talked. The only time I’ve seen many Americans together is in the restaurants because we all eat earlier than the Europeans 
The street vendors speak English to us but when we don’t answer they start in other languages. They try English first because that’s a language many people speak.
I can tell who are the Italians because they step out in traffic and know that the cars are going to stop. I follow the Italians.
We have very white, very straight teeth.
We take up a lot of space: we have wide open American faces, we are physically big, and we are loud.
We are a bit sloppily dressed.
We wear running shoes when not running.
We wear outerwear with logos on them. (Northface, Columbia, etc.)
I spent three weeks in Spain and played a sort of game with myself looking for Americans. I was wrong every time. People dress the same.
I noticed when my daughters’ high school hosted exchange students from Germany (one of whom lived with us for three weeks) that all the students were better dressed than the typical student at my daughters’ school. And we live in a well-to-do suburb.
I’m pretty good at identifying the European tourists in San Francisco or New York, based on hard-to-describe differences in gesture or posture.
Fit, clothing and accent are some signs. The Europeans I know are all multi-lingual compared to the one or two languages many Americans are limited to.
We had Swedish visitors last year and the woman always wore a scarf and the man had his polo collar turned up with a sweater worn over his shoulders. 
Looking at our photos from our trip to London in 2002, I see DH wore baseball caps that said “US Open” and the other said “USA”. Not wearing those on the next trip!
Yes, I believe scarves worn by women are MUCH more common in Europe than the U.S. In HI, scarves are rarely worn by anyone!
Sneakers!
T-shirts and sweatshirts with college names or attractions. My husband travels extensively throughout Europe and the Middle East. He always tells acquaintances (folks who he has no business ties with) that he is Canadian. Also turns his passport around when in airports so that the U.S. seal doesn’t show.
We’re the only ones not smoking.
My daughter could always tell the Americans in Europe by how loud they were. Anytime she heard kids exhibiting a temper tantrum or acting obstinate, she said they were American.
I agree with the sneakers , running shoes when not running…but I also see a lot of people in Europe wearing Northface and Yankee hats.
We were in Sweden around the time of the start of the Iraq war when America was hugely unpopular , even amongst this who generally like Americans. We took the train after landing in Copenhagen to the city of Malmo ( which has a big problem with their immigrant ghettos ) My husband had to leave me with our daughters , plus one of their friends while he picked up our van. My goal was to NOT stand out as Americans so I told all the girls to speak quietly so we could blend in …four out of five of us looked like we could had passed for being Scandinavian…my step-daughter who has ADD forgot and spoke in a pretty loud voice . I still don’t like to draw attention to myself in some places when we travel there
Shorts on women.
Sneakers are in style right now as street shoes for European women, so that’s not a good tell right now.
I haven’t been to Europe in a while, but other than the sweatshirts and generally louder and more casual clothing, I think the other thing is loud speaking, the volume. When I would notice Americans, it would be because they were louder than anyone else on the bus or in the restaurant.
Hair cuts and styles
Just read an interview that quoted some bartenders who say they recognize American tourists because they are smiling, loud, and confident. Can’t remember the quote exactly but one said something along the lines that Americans just expect you to like them. (They also mentioned the sneakers.)