I think an enlightening (and fairly current) example is the run-up to the war in Iraq. America made a diplomatic push for military and political support from pretty much all of its allies. I think it's clear that America wanted every one of them on board for overthrowing Saddam, at least politically if not actually sending troops and cash. While America has rewarded the allies that participated, and penalized those who didn't, I think that an actual empire would have excercised much more coercive power in such a situation. Major allies like Germany and France struck postures that were outright rebellious, and yet remain allies with close political access and extensive military cooperation. When an empire goes to war, support of the subordinates is not optional. To my perception, the political relationships reflected in the process leading to the invasion fit much better with hegemony than empire. Perhaps the most telling element is the expectation on America's part that the allies needed to be convinced to side with America, rather than simply ordered.
This does, of couse, leave open the position that various countries which are US allies are not necessarily components of the empire (France, for example). However, the same political dynamics played out among countries with very close, highly dependent relationships with America (Canada, Mexico, New Zealand). Also, it's hard for me to think of a coherent border for American empire that wouldn't include Germany, given America's preeminent military role in the area. But, again, the posture of the German government in the Iraq run-up does not fit with my impression of an imperial relationship.