The previous reply mentioned checking using feeler gauges. Here are the tolerances: The front gap between the cylinder and forcing cone is
0.004" - 0.010". The rear gap, which determines head space, depends upon the revolver you're checking. For 38, 357, 41 and 44 it's 0.060" - 0.068" but if the 38 or 357 has a counter bored cylinder it's 0.012" - 0.018". This can help to check Endshake but if you not trained in proper repair which can sometime be accomplished with the proper size bushing or stretching the barrel of the yoke, take a revolver with this problem to a good gunsmith or send to S&W.