I don't think you could tell by looking at the 2 I've fried. If you held it under your nose you could though! Both of mine were just barely fried, people who have hard core fried them, by dead fans etc, might have some distinctive marks.
i have a 800 tbird that isnt fried but i think it got VERY close one time! hehe, it has a barcode on the bottom of it and has a burn ring in it!!! hehe
the stupid people i got my computer from a long time ago when i didnt know a lot, they had the CHEAPEST HEATSINK IV EVER SEEN on it!!!!, and a 50mm fan!its like ones iv seen on a p66 with a fan! its really hopeless!!!
I smelt that alot, I believe my fan in the PSU failed, and it was smelling like burning! Luckily I was on the computer, and well I turned it off, at that time I knew nothing about computers, then my dad brang it to these people who replaced the fan.