Well going from the comments it looks-
Females: Nonchalantly cool with everything but shitshitwhyamifeelingthis inside, and basically acting like the less I show I like you the more I freaking do
Males: I'd put my hand on a blade for ya, I'd jump in front of a train for ya...
Then you have those: -goes to the internet and analyses love trying to see if they're obsessed, infatuated, truly in love, hungry or just sleepy-
For me personally and thru experience, its:
When you'll keep thinking of the person in whatever situation you are in. Really, that is the first symptom. Eating dinner, doing homework, even reading a book - it sorta drives you up the wall after some time. Like what the hell has he got to do with this. Around the person, you won't show it if you don't feel like asking the person out - you'll act more normal than ever. So extremely normal it gets abnormal.
Sometimes, you find it disturbing and sort of push it away because you either don't want to acknowledge you like someone, or that person doesn't seem good enough, or that you don't think you actually 'like' that someone.
You might even find something, anything else to distract you from it. Like immediately go out with someone else. Or put some seriously cute idol you aren't even particularly obsessed with as your wallpaper.
Then one day you'll decide, heck I like you.