One of the most obvious is the roll marks. If the edges look rounded and not sharp and crisp the gun has probably been re-blued. The depth of the roll marks will also look shallow.
If you've found one that you're considering buying, I'd find a picture on the web of a new one, and print it out. Then take it with you and compare it to the one you're looking at.
Is this gun at a dealer or is it a private sale?