"You'll know it when it happens" is a very low value situation because it doesn't screen out false alarms. If you're in love, you "just know it", but if you're not, you might mistake anything as mundane as motion sickness for love.
In fact, from that point of view, it can be said that being able to recognise something only by subjective measures when it happens to you is worthless. I imagine there are some criteria by which we judge these things, but they're layered thick so that it's difficult to articulate them.
Mokalus of Borg
PS - Also, for a subjective effect, it's difficult to get an objective viewpoint.
PPS - Probably impossible.