Well, loving someone is more of a general thing. I love all my friends. I love everyone here. It's usually applied toward friends and family. Being in love with someone is a romantic statement, and it's often a much more concentrated feeling.
How to tell the difference? Well, I think that if it feels like every cell in your body can't live without that person (and I'm not talking about lust here, folks) and it's literally painful to be away from them, then I think you're in love with the person. If it's more of an "I miss him and I wish we could hang out today... oh well" thing, then it's more of a general love.
Love drives me crazy, you know? Sheesh. So hard to define!
Being in love is an intoxicating feeling that tells you that you love someone. It's a shallow sort of love... only a feeling.
Love is something that you may or not know about. It's deep, but hardly ever really comes to the surface until you realize how much you actually love it/him/her. And it can and will destroy.
Ubi caritas est vera, Deus ibi est.
"The mark of your ignorance is the depth of your belief in injustice and tragedy. What the caterpillar calls the end of the world, the Master calls the butterfly." ~ Richard Bach
I disagree with that. Actually being in love isn't shallow. It's the deepest you can get, at least in my mind.
And as for the difference between the two... it's hard to explain. Like, I love my ex still, but I'm not in love with him anymore. I love my friends, but being in love with someone, it's like... you want to talk to them every waking moment, want to be with them all the time... And that sounds totally cliched and out there, I know... but it's hard to explain.
I don't think there's any way to know whether you're in love or just loving until your old enough/mature enoungh to have seen something that makes the difference clear as day in your mind.
The robbed that smiles, steals something from the thief. ~William Shakespeare, Othello Boo. SPEW is watching.