DS2 is studying abroad right now but not in Europe. He took very non-descript clothes by design. No labels. And he bought sandals to blend in with the locals. He rarely wears his Nikes. He’s dark enough that he actually gets mistaken for Indian in some parts of the country. I’ll have to ask him whether he thinks he louder than others. That’s interesting.
I don’t think you can “tell”. When I travel, people assume I am English or Australian unless it is the UK. They seem to be the only one who can tell!
Honestly, it doesn’t matter. Terrorists are after Westerners, not just Americans.
^ Just to clarify, terrorists are killing way more people in their own backyard than anywhere in the West.
To the OP: to echo others, my British family says its our volume more than anything.
I’m not sure they care so much about any nationality – they want body counts and to scare, maim and terrorize. When we have traveled, we have people unable to tell our precise nationality, other than that weare visitors because we don’t easily know our way around. I don’t wear shorts at all when traveling and stick mostly with dark colors and no brand names anywhere. Our family has never liked brand names and being walking billboards anyway. I and H and kids are slender and mostly quieter. We have always avoided crowds. H likes to always blend in whenever possible.
But tourists are not headed to their backyard. I was answering the question is all.
I’d say we are louder than the British but not as loud as the Italians.
@emilybee is right on. The Europeans are the ones smoking.
My daughter studied abroad in Europe and she and her friends were told that Americans tend to slam the door to a taxi compared to Europeans. I thought that was funny…and took care to close the door carefully when there. 
Last time I was in Germany it was that we drive less erratically on small town streets.
Spanish, Dutch and Australians can be quite loud as well.
Really, as soon as you open your mouth everyone will assume you are from the USA. I think it’s naive to think you can blend in. Your risk of being a victim of terrorism is very low. Take necessary precautions, same as you would traveling in any major cityeven in the US, treat people you meet with respect, and trust that 99.99% of the people you will meet have goodness in their heart.
When we’re in Europe, we try to dress like the Europeans. In other words - better than here 
No T-shirts with things written on them, husband wears real shirts, no shorts and no tennis shoes (although it’s true the Europeans wear them too.). I tend to wear skirts, and add nicely tailored tops if I’m wearing pants. When we traveled with our kids, they also dressed “up” – and daughter would often get a chic French haircut; something she totally enjoyed. Husband and I speak French, a lot of Spanish and enough Italian to get by - which usually means that Europeans think we’re European (weirdly, a few times we were taken for Norwegians for some reason) since Americans have the reputation of rarely speaking ANY foreign languages.
But I also agree that we tend to smile a lot. And somehow take up more space, even if we’re not actually fatter than the locals.
Remember, the chances of being in an auto accident or assaulted are MUCH, MUCH greater than the chances you will specifically be a victim of any terror attack. Horrible events grab headlines but there is much more domestic violence, DUIs, auto accidents and assaults than these headline-grabbing horrific events.
I was told in Israel that I was easily identifiable as an American by blonde hair, straight teeth. I also think running shoes, casual dressing makes one identifiable. Just wear all dark clothes, nothing casual, no tennis shoes. Dye your hair and don’t smile and you’re set!
There are lots of things.
-Super clean bright white sneakers worn by anyone over the age of 20 or when not actually engaged in sporting activity (especially when wearer is also obese).
-Being really really loud and having absolutely no idea this is the case (especially when an entire restaurant or train carriage is giving the look of death at the speaker).
-Stone wash and/or baggy jeans. Skinny jeans in dark colours are more usual in Europe.
-Fanny packs.
-Bright colours in general. If you’re not on the beach, bright coloured clothes are less common in Europe.
-Adults dressed like teenagers.
My comments are based on Europe and may not apply to other areas.
I honestly wouldn’t worry about it because it will be obvious you are a tourist any way, just from your location and activity. Everyone who goes to the Eiffel tower for example is a tourist. Including French tourists. So none of those people are likely to be wearing business suits and it’s obvious they’re not residents on their way to work in an office.
It’s almost impossible to be totally invisible as a tourist (you would have to never speak in public) so I would recommend just trying not to look like a clueless and lost tourist. I know myself I have become really pale from living in Britain. If I visit somewhere warm I really stand out now due to lack of tan. I don’t do anything about this, but I could. I just try to act confident, like I go there all the time.
I know we are discussing terrorism here, but you are millions of times more likely to be pick pocketed. Try to look like a confident and regular traveller (burn that fanny pack). 99.9% of people will probably know you are foreign and won’t care.
My daughters did study abroad programs in Africa and southeast Asia. They stuck out physically and I think they benefited from learning what it feels like to be an ethnic minority and to be the “other.” Both places were probably safer or as safe as their college towns as far as crime was concerned, but both were more dangerous as far as transportation was concerned (for passengers in motor vehicles and on motorbikes and for pedestrians).
I believe it’s not the outfit but the choice of words. For example, Americans are identified by words like “mom” versus “mum” and our accents.
No one wears those anymore… I hope.
sadly, they do
Actually, fanny packs have had a resurgence among young people. It’s been hipster boho the past couple of years.
http://www.details.com/story/valentino-fanny-pack-mens-fashion-week-paris-spring-2016-pictures
If you google, you’ll find many references to this trend. They are actually practical, especially with larger smart phones and form fitting clothing.
Yes! I went to a boho art gallery on First Friday and was amazed to see the coolest-looking (artsy) young woman selling fanny packs she designed and made. See, it’s true. Everything comes back.
Bad news: “Dickies” are back too.