I always thought of it like this: If you can describe it at length, you're not in love. If there's simply no words to properly describe it, you're in love. <3

There is no way to tell except by your own standards, unfortunately. But, to help you out, when I know I'm in love with someone (my girlfriend now), I certainly can't stop thinking about them. I also noticed that my heart tends to ache a bit when I miss them alot (it's a good ache).

However, it's different with everyone. Some may fall quicker than others. But remember, only you know that you are in love. It's kinda like what my girlfriend's friend told her, which I disagree with:

"You can't miss them if you haven't met them yet."

Certainly, it's not true. If your heart aches longing for that person, you do miss them and you do love them. Hope this helps.

I think @xraindropsonroses has the right idea. But it's also important IMHO that the other person shares the same feelings with you. You shouldn't love someone who doesn't love you back because it's just bad business. People always gloss over the part where the other should reciprocate the feelings, but it's equally as important, I think.

And as an aside, you love someone when you can trust the other person with your deepest and darkest secrets and know that they will keep it safe for you, and vice versa. You're in love with someone when you feel like you would go to the furthest lengths in the world to make your significant other happy. And your significant other also makes you happy. Love and happiness should be synonymous

It's when you can deal with him when he's being a complete asshole.  It's when you know you'd die for him if someone came in and pointed a gun right at him.  It's when you know with certainty that this is the one and there just really can't be anyone else. Ever. You just know.