Like BobK48 said, open the cylinder, and look inside the frame where the crane/yoke go into the frame. There will be a number, and sometimes the number is followed by a dash and another number. The first number is the model number, and the second number (if there is one) is the iteration or sub-model number of the piece. Each iteration is for either production runs, or because the factory decided to change some small part or manufacturing method on that run.
For example a 19-2 would mean it is a model 19, second iteration. A 27, would mean a model 27 first iteration. A 27-4 would be model 27, fourth iteration.
Some times the serial number, or part of the serial number is also stamped inside there. I have had revolvers that had the serial number stamped there, and others that didn't have a serial number inside the yoke/crane area.