Click on the Apple in the top left-hand corner. Click "About This Mac" A box will appear saying "Mac OS X" and then underneath it will be the version, if it says 10.6.x then you have it.
If you've just bought it brand new, it's snow leopard.
If you are this insecure about it, click the apple logo on the top left, click about this mac, the version of mac os X should read 10.6.x (x being either 0, 1, or 2)