Time to find a new roommate. Sorry he turned out to be such a thief and a liar. I guess from now on, carry receipts with you.

Did you pay in cash? If not, get statements from the bank or credit company. If you bought it in a physical store, which is what it sounds like, talk to the manager at the store and see if they can pull video of you purchasing it or pull a copy of the receipt. Then take the bastard to small claims court if you have to. If you paid in straight cash... don't ever do it again, because stuff like this happens, unfortunately. I hope the bastard gets kneecapped.

Must suck to live with a diabolical genius!

first things first, find a new roommate! Next, did you buy it with a card? Very easy to prove its yours if you purchased it on a card! bank statements and the last 4#'s on the receipt will match your card. If you purchased it with cash get security tape from the place you purchased it, they'll have tape of you purchasing and walking out with the laptop.

Didn't you put a password on your laptop?

If you paid with a card, verify it's yours by showing them the last 4 digits on the card and compare with the receipt. Should be enough proof that it's yours

If you pay my credit or debit card, then you could easily prove that you bought it.

If you paid with credit card, you can easily prove yourself

Show them your bank statement!