How to recognize when a crush turns into obsession?

It come overboard when you can't ever take your mind off them. When you go to stalker levels. When you force them to do things. Stuff like that.

 

Staring at someone constantly and making him or her uncomfortable.

Texting, emailing, IMing or calling all the time, especially, if you don't get a response.

Coming on to someone repeatedly after they have told you they aren't interested.

Constantly asking someone to hang out, even if he or she has said no.

Feeling really depressed, and worthless if your crush doesn't return your feelings.

Not being able to be interested in anyone else.


Source: I'm Gay but I Have a Straight Crush - Advice for Teens

 

Also, usually it's a phase that runs its course. Just don't let it turn you stalker-ish!
Process the fact that it's a visceral reaction, recognize that you're building castles in the sky, have fun imagining your life with him, and so on.
In real life - try to become friends and see if the attraction survives. And then see if it's reciprocated. Only then should you move forward.
Otherwise there's too much risk of heartbreak :-/

---------- Post added 5th Jul 2016 at 02:37 PM ----------

Yes, it could be borderline obsessive. Think of it as a hurricane or a tsunami. Irrepressible when you're right in the middle of it. But give it time and it will wash over you and eventually dim. Usually could take weeks to months though

---------- Post added 5th Jul 2016 at 02:37 PM ----------

The key is not to let it get the better of you.