What's the trivial difference between the 15" and 17" versions of the MacBook Pro?
In the left corner of the top menu, pick the apple menu and select About This Mac.

In the About This Mac panel, click the More Info  button. if you hover your mouse on Hardware you can see the general info about your macbook pro such 15" or 17".



As Dori said in comment above solution for before 2007 now choose Graphics/Displays from the left column, it does display the screen resolution.