study... research
I've gotten myself into this predicament, and it isn't a lot of fun. Read the many articles on how to check out a revolver. You may have to go under specific brands, and use that information to generalize.
In a nutshell, the more you know, the better prepared you will be. You can take a lead ball, larger than the chambers, press it into the end, and see if the other chambers line up with the circle your pressure made from the first impression. All chambers should be exactly the same size. Feeler gauge to measure cylinder-barrel gap, smooth action? Is the cylinder locked in position when the hammer is cocked? Look for frame cracks and erosion where the cylinder chamber gap projects gases. Check the bore for smoothness, corrosion, nicks in the crown... gosh, the things to check out are next to endless. Some people will actually "slug" a barrel (force a lead ball through the barrel), and check the diameter, and notice if the lead ball goes down smoothly, rolls freely, or hits tight spots. Get all the reviews on the models you are looking at and take criticisms seriously. Good luck!