My hands are all messed up

oh.

If you have to ask how to tell, then you're not?

upside down

No, wait a minute...

In fact if you ever suspect that you might be, you convince yourself that you're not and end up dumping every girlfriend you've ever had because you feel it would be futile to do otherwise.

and everything somehow relates back to them and you get depressed when you're not with them. Also, silly things like liking the exact same pizza toppings and listing them in the same order at the same time making you think you are soul mates.

when everytime I walked past a mirror, I just had to stop

and sad when you are not.

At least, that's what it feels like to me. Like your heart is going to explode when you see them. Or something.

.. like you can't stop grinning all the time, and thinking about them makes you feel happy. I think there's like a security with it as well.. with lust you feel a bit more on edge with someone (not always in a good way) but 'love' I think of as being more easy going.
As with everything, there's different types/stages and everyone'll have different takes on it.
I've been seeing a nice young lady for about 2 months now and it's only recently I've started feeling like that. :)

only you can know

you insides are being eaten away repeatedly by an unknown force, knawing at your very being; when you lose interest in everything except the object of desire; when life seems meaningless, pointless and without purpose; when you convert the celllar into a comfortable but well-secured room just in case your love is not readily reciprocated.

you think about them all the time and everything is basically about them, like say you see a portuguese tourism poster and it has cristiano ronaldo on it which reminds you of them because they try and do their hair like him which sounds vain but it looks nice! and when you have pizza because you had pizza when you went out in london and he peeled all the cheese off and it was disgusting. and all the songs that came on in all of the shops were about them. and all the conversations you had were conversations they should be in on because they'd LOL SO MUCH if they heard what that person was saying. and you were in derby train station and there was this poster saying derby was the gateway to the peak district which reminds you of them because they wanted to go there with you awwwww. and whenever possible you have spaghetti bolognese because it's their favourite even though it's not your favourite but you have it anyway because eating it reminds you of them. and you start liking songs you used to hate but they like because they like them. and YOU DREAM ABOUT THEM

...then chances are you're probably not. You just know. There's no pondering involved.