Apple Menu -> About This Mac -> More Info
But I think you mean the model identifier?
Scroll half way down the page to that large table that says "*-inch Configurations"
13-inch: http://www.apple.com/macbookpro/specs-13inch.html
15-inch: http://www.apple.com/macbookpro/specs.html
17-inch: http://www.apple.com/macbookpro/specs-17inch.html
And one other "stock" configuration that is not listed on Apple's website - the 15-inch i7 with High-Res Antiglare screen - MC666LL/A
You have to go online for the i5. I think the only stock model with the AG screen is the 15" i7 (the 17" may have the AG screen as a stock model as well, but I have not checked)