Ask Max: How do I know if a Macbook has Leopard installed?
How do I know whether a Macbook Pro has Leopard installed?
If you’re referring to before purchasing, on every Powermax.com New Mac product page there is an info box listing: Features, Detailed Specs, Rebates, Related Items, and Recommendations. The Detailed Specs section contains useful information about the system, including the Installed OS.
On Pre-Owned Mac systems the Operating system installed is part of the main description, right above the price.
If you mean on your computer itself, simply go to ™About this Mac™ in the upper left hand corner (under the Apple logo). There, it will tell you a little bit about your Mac (more if you select ™More Info™), including what version of OS you’re running. Leopard is OS 10.5 (often following by another digit indicating what update has been run on Leopard, for instance, 10.5.5 is of course more up-to-date than 10.5.4), so if you are showing 10.5.x, you have Leopard.