Leathermech,

In a nutshell, you already had the right answer before you posted. Your thinking is correct. The cases and primers, whether .38 Spl or .357 Mag, will usually function properly beyond what some revolvers will handle. I'm not sure that the top strap is the first to go, but you have the general idea. Reminds me of a redneck torque wrench - tighten until the bolt snaps, then back off a quarter turn.

You've heard a few suggestions, and certainly you shouldn't expect better velocities than factory, but powder manufacturers' reloading manuals, or even bullet manufacturers' (minus Speer), are the way to go.

Pressure signs? Top strap is as good as any.