Hmmm...this is tough one, it's actually easier...
to explain how to service a headshock!!!

But seriously, it might be a thunderbolt, it might not. How do you know? Well, I think that you'll know when she becomes the 1st person in your life. What I mean by that is that in every asspect of your life your first consideration is her, not you, not mom and dad, not the ride your going to miss by going shopping with her nor any of a thousand other things. When you worry about her pleasure and happiness rather than your own. If you can't live without her, want to be with her every possible minute, would rather be with her than anyone else in the world. Then I'd say your in love. It's tough to tell the difference between loving somone and being in love. The best illustration would be, you love your sister, brother, mother, father, your best friend, but you are not in love with them. Being in love with someone goes WAY deeper than that. When you are in love with someone, that person becomes your world, everything in your life revolves around them, or at least it seems so. If that's the way you feel then you are in love.
Like I said, the realization can come suddenly as some here have said. Or it can just dawn on you slowly. I know when my Wife and I were dating it just kind of dawned on me slowly that she was the one I wanted to spend the rest of my life with. We had been going together for about a year. I had to take a trip that kept me away from home for about 2 weeks. During the trip I was restless, bored, and just couldn't seem to relax. When I got home and she met me at the airport I was never so happy to see someone in my entire life! I had NEVER missed anyone as much as I had missed her while I was gone. I knew that I loved her and had said so, but until that time I really didn't know that I was IN LOVE with her. About two weeks later I asked her to marry me. The rest is history, and 28 years later she is still the person I would rather be with. Don't get me wrong, we've had our rough spots and trials. But if you really are in love with someone, you will work your ass off to make it work, because the person you are in love with is more important to you than your own life!
How will you know for sure? Well, really you won't. Not in your head anyway. Men are more logical, and need to "sort things out" and be sure. But you know, being in love with someone is the the most illogical thing in the world. There is no explaining it in such a way that is suitable to the male way of looking at things. That's why alot of guys have a tough time expressing love. It doesn't make sense! That's because love is a heart thing not a head thing. It's 100% pure emotion, it makes no sense, and is illogical as it gets. The bottom line is, give it a little time, you'll figure it out. Just don't try to reason it out, it ain't possible!!!
Anyway, that's my take on it. Sorry for the long post, but like I said, it's easier to tell you how to service a head shock than it is to explain love and being in love!