It takes a lot of looking at guns and, I admit, a bit of instinct. And, yes, MountainBear is right; if a reblue job is done well, especially by the factory, it is pretty hard to tell. I have a K-22 that I bought in 1957 and sent back to S&W in 1985 to have the firing pin (frame mounted) replaced. They wrote the dealer asking if I wanted the gun reblued for $17. Of course I did!
So what did they do? Well, they stripped the frame, then rebuilt the gun using all new parts. What I got back of the original gun was the frame and the grips; everything else was brand new. Anyway, they reblued the frame and blued the new parts; there is no way anyone could tell that reblue job from the original finish except by the S&W refinish mark.
That was the extreme. Most reblue jobs are just not that good, and I dare say I can spot at least 98% of them, though not always from a photo. As an example, even when a polisher is careful to polish the sideplate and crane with the frame, the wheel will drag enough to make the lines more visible than in a factory finish. One area that is hard to polish without some evidence is the S&W or Colt trademark on the side of a revolver or slide of an auto pistol. Those markings are finely stamped and even the lightest polish will remove the displaced metal the factories (deliberately) leave in place.
Just FYI, big factories do not polish guns on the usual wheels. They have hard wheels specially shaped to the gun. For example, S&W has a hard wheel for each frame size that is shaped to polish the bottom front of the frame, the trigger guard, the inner curve of the grip and the front of the grip all in one pass. No gunsmith or restorer has that kind of equipment.
Jim