How can you tell if your Mac has os x?

Click the Apple at the top left of your screen.

Click "About This Mac"

It will say

Mac OS X

Version 10.X.X

X will refer to which one you have

10.0 Cheetah

10.1 Puma

10.2 Jaguar

10.3 Panther

10.4 Tiger

10.5 Leopard

10.6 Snow Leopard

10.7 Lion (Coming Summer 2011)

The last X in 10.X.X refers to the last time software update was run/released by Apple