How to know if the iPhone 4 I purchased is stolen?

I bought a iPhone 4 off a guy who said he was selling it for a friend.
This guy went to my secondary school.
I spoke to my mate and he said the way to tell if it is stolen is to put a sim card into the iPhone and see if it is blocked.

Is there any other way to tell if the iPhone is stolen? I don't have a sim card for it and I don't want to sell it on ebay if it's stolen, I'll report it to the police right away.

Added (1). It only came with a charger

You can look up the serial number on the apple.com website

Smart people with smart phones that get stolen block the EMEI number and the entire phone is blocked off.

Sounds iffy to me

Did it come with a charger and headset and box if not its definitely stolen