There are easy ways to spot Americans overseas: the overhanging gut, the baseball cap, and throwing tips around everywhere.
As OTB points out, there are some doubts on who and what to tip in the U.S., but in most overseas countries it is pretty easy: it generally isn't done. Hotels, restaurant and service establishments in Asia add a 20% service fee to the bill - in addition to taxes - to take care of what would be considered tipping here. And taxis, doormen and the like don't expect tips - except maybe when Americans show up. I have had Japanese bellhops hang out in my room wondering if the American knows whether or not to tip. I simply smile, bob my head and say arigato, letting them know that this gaijin knows what he is doing.
But I have been around Americans who don't know what they are doing - and not used to NOT tipping - in a non-tipping country. While in Israel, one of my colleagues was told by a local point blank: Do NOT tip the taxi drivers. It is not done here. I watched over the next three days as this guy kept digging into his pocket for extra shekels for taxi drivers. I kept reminding him and he kept doing it. When I asked him why, he mumbled something about feeling guilty about not doing it. He was trained that way.
I don't have that problem, which is one reason I also don't have a problem not tipping in places in the U.S. when it is inappropriate, like at Starbucks, when I get take-out, or when I get crappy service (but will give out over 20% for stellar service). I just don't have a guilt complex when it comes to giving out my hard earned dough and believe tips should reflect the service given, not something that is done automatically.