how to id turbo?

hey everyone looking to buy a t25 or a t28 turbo and having trouble telling the difference between them. Other than a a/r ratio is there any pics or any numbers to tell what turbo’s what? How can you tell what turbo’s from what motor (nissan) without taking it off the car yourself.

Anyone know a link to some pics? I tried google, ebay, msn this forum and its hard to find some good pics. All info would be greatly appreciated.