If it was fired a few times and then professionally cleaned and polished, there will still be a few signs left, like maybe a trace of flame cutting on the frame above the forcing cone. Those lines are hard to get rid of. If it was fired with .38 caliber rounds instead of .357, there may be some signs of carbon buildup in the chambers.

If the gun was sold in the early 70's with target grips, the grips will most likely have the extractor/football cutout as opposed to the later speed loader cut. If the gun was used, then the grips will probably have a slightly worn spot where the cartridge rim hits the football cut on ejection. If the grips were factory fitted, then they will be flush at the top back of the grip, and also at the bottom of the back strap. If there is some space at the bottom (heel of the backstrap), then the grips were added later.

Someone can probably correct me if I am off on some of that.