I had a buddy that spent three months in Europe. He's Canadian, but has been in the states for 15 or so years. I was curious to see if he noticed if nationals from the countries could pick up on his nationality. However, on account of his actual nationality, I had to scour the confines of Reddit, and see what people from other countries think of us as Americans.
Flickr, FightHIVinDC

  • If you say, "That's funny" instead of laughing audibly.
  • On campus in South Africa the female US exchange students uniform: tiny shorts (regardless of body shape) hidden socks and trainers, hoody (usually branded America college variety), back pack, hair tied up and a water bottle. Can be found frequenting coffee shops with free wifi. Also can be found in North Face jackets, Uggs. If it's cold, yoga pants instead of tiny shorts.  
  • You may be friendly to the point, that others become suspicious of your intent
  • We (Americans) describe distances in driving time, as apposed to miles or kilometers. 
  • "I was like "oh, my god" It seems to carry for miles. "But like, are you KIDDING ME? I can't even...."
  •  Europeans say "Uhhh" where Americans will say, "Ummm."
  •  You may call someone "Honey/Darling/Sweetie." Bless his heart.
  •  "Foreigners think all Americans are extroverts because those are the ones who travel and talk to people."
  • "I'm English, but I've lived here for 14 years. It's pretty obvious just from your demeanour. Americans generally are more confident in the way they present themselves, most other countries tend to be more reserved. Walk into a room full of different nationalities, I guarantee the American person will be the first to introduce themselves. It's a confidence thing, and I admire it." 
  • "Speaking as a former barman, or "bartender" as American customers would say... tipping! No British person will ever tip a barman. I'll occasionally get bought a drink by drunk ladies or gents, but Brits actually giving me money for doing a job that I was already being paid for? Never happened. I would listen for American accents (which were easy to hear due to their natural loudness) and immediately serve them next."
  • "Being mannerly and polite and calling you "sir". Americans make great tourists because they're generally looking for fun and not afraid to spend money."
  • Bonus points: "I'm an American. I was in Italy a few years ago and an American couple stopped and asked me for directions. I gave them the information, and then the husband said "Your English is very good, where did you learn to speak it?" I'm from the midwest, so I said "Minnesota". His wife asked me what I was doing in Minnesota, and I said "growing up". I don't think they ever understood."