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The first check is for barrel/cylinder gap, and you check with a automotive feeler gage.
Press the cylinder to the rear and hold it there while you gage the gap.
The correct gap for a S&W is "about .005" with a maximum allowable gap of about .010".
Then press the cylinder forward and hold while you gage again.
If you notice a big difference between the two measurements, this is an indication that the gun has developed end shake, or wear that allows fore and aft movement of the cylinder.
S&W is in spec if the difference between cylinder to the rear and to the front is no more than "about" .005".
Anything over that requires repair.
Another thing to check is the forcing cone, which is the "funnel" in the rear of the barrel.
Unfortunately, you'll need a pistolsmith who has the proper plug gage to check that.
To check timing on the S&W, simply cock the hammer very slowly.
The cylinder locking bolt should drop into the cylinder locking notch with an audible "click" and the cylinder should lock up BEFORE the hammer reaches full cock.
If in doubt, send the gun in to S&W. They'll give it a full check-up and do any necessary repairs for a reasonable price.
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