Find some of Richard's very clear photos of the S&W logos on the right side of the frame. Compare those to yours. If it's been re-blued there's a good chance the details will be smoother and more washed out in yours. Also look for wear on the inside of the screw holes - original blueing will have very sharp edges to the holes.

I am very paranoid about re-bluing and re-nickling. I recently passed up on a legitimate nickle Model 19 because I thought it had been refinished. I lost a great deal and could have just asked for a clearer photo of the s&w logo. But another one is just around the corner. As cool as these guns are, there are still lots of them around in nice condition.