Dill: good question. First thing I'd want to know is "how likely is this a stretched frame?", that being the worst sort of wear.
If it's an older gun, or esp. an alloy frame, that becomes a serious concern. You also want to think about whether it's a gun at the "edge of it's power curve" of frame strength to caliber. The M66 is unfortunately such a gun - a lot of hot loads can stretch a stainless K-frame S&W. The K was originally a .38Spl platform, slightly beefed up for the .357, but still kinda marginal esp. in stainless.
A gunsmith could tell you what's up. If the frame isn't stretching, it could probably be shimmed up pretty easily. All that "the test" can really tell you in this case is "skip that gun unless you have a real good reason otherwise". The danger on that gun is that the cylinders are lined up with the barrel end to end, but not "in line"
- there may be a slight effective "bend" in the bullet's direction from cylinder bore to barrel.
ADK: Sam called it right. I'd still test rotational slop at full lockup on general principles, but I'm not sure it matters on an N-frame. It makes a difference on some guns, not others. I just specified "test at full lockup" to cover any possible situation.
SimonBarsinister: I asked over on the semi-auto forum if anybody could do an equivelent post like this. Short answer I got: not possible, way too many variations. You just have to know the particular gun type and do what you can. They're actually simpler; most problems happen at the feed ramp or magazine.