@The Illustrious Master Dravis:
Thanks for posting that exchange. The problem with defining/testing love is the same problem that you have for defining/testing any emotion. You can accurately identify a range of feelings, usually associated with a corresponding set of behaviors, but you cannot point to one specific subset of either and say that it accounts for all of it.
As for being able to identify specific emotions in others, well, that's complicated. As social creatures, most of us are well adapted to recognize certain cues, however those cues can be faked convincingly. To that end, one would have to accept that yes, it truly is impossible to determine 100% that someone is feeling something. However that doesn't mean that we cannot accurately deduce whether someone is genuinely feeling the emotions that they say they are feeling.
See also: Theory of Mind