you just have to remember how to tell the difference between love and obsession. it's a real thin line.
You would know. He would make you uncomfortable by trying to be around you all the time and talking to you all the time.
Obsessed? You'll know because he'll talk about you to other people. Contrary to what is popular pseudo-psych males talk a lot more about whom they like and specifically focus on that person and they do it a lot so if you're not painfully oblivious you'll hear about it.
Other than communication obsessed people will pay extreme attention to detail in relation to you and will be fairly obvious of their obsession regardless of shyness or not even going so far as to "closet" aid you in various ways trying to be part of your life by doing kind little things or helping you without your permission behind the scenes as appropriate ranging anywhere from putting a good word in for you here or there to helping you in your studies and at work to succeed in whatever your endeavors are without being intrusive.
Honestly at that point body language is pretty pointless but then again I presume you really meant obsession versus interest as the two play out entirely differently.
If he starts stalking you...then he's obsessed. :/ lol
Extended eye contact.
Studering when speaking to you.
Smiles at you, when you talk.
He keeps a hair doll he has made out of your lose hairs and keeps it with him at all times...
90% of the time, body language often tells the truth.