I've only been in love once. And it started slowly, but once it began it escalated quickly. My love became very intense. Like JCG, I think about him all the time, want to know all about him, grin a lot (over the dumbest things, just thinking about him), want to be around him a lot and want to make him happy. Compared to my feelings for other men I've known/dated/etc., I find that I care more about how he feels, how things affect him, how his life is in general. Things that bother me about other men don't bother me about him. That could change over time though.
We're similar in a lot of ways, which is odd because people might assume we're very different. We have similar backgrounds/upbringings/parents and I can see a lot of myself in him (and vice versa). We react similarly to some situations, which can be maddening but comforting too. I don't know a lot of people like me, so meeting a man like myself is interesting...Our senses of humor are about the same too.
I don't know, if you just KNOW if you're in love. I think a lot of people confuse love and lust, or even love and codependency. Perhaps if you've been in a relationship where you've confused love with something else, it's easier to know the real thing when it does happen though.
That's how I'm experiencing being "in love." But it means different things to different people. So others can call it what they want.