Most popular right now is skinfold - when done properly, best you can get is +/- 3-5%. There are simply some inherent flaws in the method that make it innacurate to some degree on most people.
The old "gold standard" was underwater weighing - that could be manipulated by the subject, however, due to the portion where the subject blows out as much air as possible. It's being replaced by DEXA as different institutions get the equipment - it's extremely accurate, I believe to 1% or so. MRI can be used as well, same deal, as you can visually "count" the fat present.
Bod pod is a new technology that uses changes in air density while you sit in it to measure - it's very accurate (not sure about the exact SEE), but not quite to the level of DEXA or MRI (though easier and less expensive).
Ultimately, though, the mirror is as good a judge as anything.