What's the trivial difference between the 15" and 17" versions of the MacBook Pro? What can I ask on the phone to a Mac user who has a MacBook Pro (Alu), but doesn't know if it's a 15 or a 17 one (and don't have a tool to measure the screen)

What can I ask him to look at to tell me what version it is?
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.