If you mean visual difference, for identification purposes, then you'd need to remove the heatspreader to see it. The die core of the Winchester is about 40% smaller. If you are trying to identify which processor core you have, it'd be best to check the OPN code on the processor. For the Newcastle cored 3500+ that is ADA3500DEP4AW and for the Winchester core 3500+ it is ADA3500DIK4BI
Yup, I was just wondering because i'm buying one but wasnt sure which to choose from, I think ill get the 90nm because it has a "Enhanced by SP2" sticker on the box lol.. nah but I guess since its newer and a smaller core might as well get the newer technology it works like the other 3500+ no compatibility issues.