Q: I am a college graphic design student looking to buy a Macbook Pro, but can't tell them apart.

I need to buy a Macbook Pro for school and I'm planning to buy one refurbished. I want to buy one from the generation before the most recent one that was released in December, but I can't tell them apart. How exactly do I determine the generation I'm looking for?

PS I don't want the macbook with retina display, because it doesn't have a CD disk drive.