Check the cylinder play.
1) With the gun UNLOADED (check for yourself!), close the action.
2) Thumb the hammer back, and while pulling the trigger, gently lower the hammer all the way down while keeping the trigger back - and KEEP holding the trigger once the hammer is down. (You've now put the gun in "full lockup" - keep it there for this and most other tests.)
3) With the trigger still back all the way, check for cylinder wiggle. Front/back is particularly undesirable; a bit of side to side is OK but it's a bad thing if you can wiggle it one way, let go, and then spin it the other way a fraction of an inch and it stays there too. At the very least, it should "want" to stop in just one place (later, we'll see if that place is any good). The ultimate is a "welded to the frame" feeling.
Check the cylinder gap.
4) Still holding the trigger at full lockup, look sideways through the barrel/cylinder gap. If you can get a credit card in there, that ain't good...velocity drops rapidly as the gap increases. Too tight isn't good either, because burnt powder crud will "fill the gap" and start making the cylinder spin funky. My personal .38snubbie is set at .002, usually considered the minimum...after about 40 shots at the range, I have to give the front of the cylinder a quick wipe so it spins free again. I consider that a reasonable tradeoff for the increased velocity because in a real fight, I ain't gonna crank 40 rounds out of a 5-shot snub .
If you're eyeballing it, you'll have to hold it up sideways against an overhead light source.
Checking the timing.
5) You really, REALLY want an unloaded gun for this one. This is where the light comes in. With the gun STILL held in full lockup, trigger back after lowering the hammer by thumb, you want to shine a light right into the area at the rear of the cylinder near the firing pin. You then look down the barrel . You're looking to make sure the cylinder bore lines up with the barrel. Check every cylinder - that means putting the gun in full lockup for each cylinder before lighting it up.
You're looking for the cylinder and barrel holes to line up perfectly, it's easy to eyeball if there's even a faint light source at the very rear of both bores. And with no rounds present, it's generally easy to get some light in past where the rims would be.
Checking the bore.
(We're finally done with that "full lockup" so rest your trigger finger. )
6) Swing the cylinder open, or with most SAs pull the cylinder. Use the small flashlight to scope the bore out. This part's easy - you want to avoid pitting, worn-out rifling, bulges of any sort. You want more light on the subject than just what creeps in from the rear of the cylinder on the timing check.
You also want to check each cylinder bore, in this case with the light coming in from the FRONT of each hole, you looking in from the back where the primers would be. You're looking for wear at the "restrictions" at the front of each cylinder bore. That's the "forcing cone" area and it can wear rapidly with some Magnum loads. (Special thanks to Salvo below for this bit!)
Checking the trigger.
7) To test a trigger without dry-firing it, use a plastic pen in front of the hammer to "catch" it with the off hand, especially if it's a "firing pin on the hammer" type. Or see if the seller has any snap-caps, that's the best solution. Flat-faced hammers as found in transfer-bar guns (Ruger, etc) can be caught with the off-hand without too much pain .
SA triggers (or of course a DA with the hammer cocked) should feel "like a glass rod breaking". A tiny amount of take-up slack is tolerable, and is common on anything with a transfer bar or hammerblock safety.
DA triggers are subjective. Some people like a dead-smooth feel from beginning of stroke to the end, with no "warning" that it's about to fire. Others (myself included) actually prefer a slight "hitch" right at the end, so we know when it's about to go. With that sort of trigger, you can actually "hold it" right at the "about to fire" point and do a short light stroke from there that rivals an SA shot for accuracy. Takes a lot of practice though. Either way, you don't want "grinding" through the length of the stroke, and the final stack-up at the end (if any) shouldn't be overly pronounced.
Detecting Bad Gunsmithing:
8) OK, so it's got a rock-solid cylinder, a .002" or .003" gap, and the trigger feels great. Odds are vastly in favor of it being tuned after leaving the factory.
So was the gunsmith any good?
First, cock it, then grab the hammer and "wiggle it around" a bit. Not too hard, don't bang on it, but give it a bit of up/down, left/right and circular action with finger off trigger and WATCH your muzzle direction.
You don't want that hammer slipping off an overly polished sear. You REALLY don't want that It can be fixed by installing factory parts but that'll take modest money (more for installation than hardware costs) and it'll be bigtime unsafe until you do.
The other thing that commonly goes wrong is somebody will trim the spring, especially coil springs. You can spot that if you pull the grip panels, see if the spring was trimmed with wire cutters. If they get too wild with it, you'll get ignition failures on harder primers. But the good news is, replacement factory or Wolf springs are cheap both to buy and have installed.
Timing (test #5) is very critical...if that's off, the gun may not even be safe to test-fire. And naturally, a crappy barrel means a relatively pricey fix.
Cylinder gap is particularly critical on short-barreled and/or marginal caliber guns. If you need every possible ounce of energy, a tight gap helps. Some factory gaps will run as high as .006"; Taurus considers .007" "still in spec" (sigh). You'll be hard-pressed to find any new pieces under .004" - probably because the makers realize some people don't clean 'em often (or very well) and might complain about the cylinder binding up if they sell 'em at .002".
