Internal winding insulation breakdown which causes arcing between windings and poor performance cannot be checked with a VOM. You can check for gross leakage to the case and measure the comparative resistance values of the primary and secondary windings with a VOM, but the only way to see if it actually works is to connect it to 6 volts, a capacitor, and a lead to make/ break like the points. You should be able to get at least a 3/8" long blue snap on each break.
The coil may still be susceptible to heat breakdown. Running it hot for an hour with a greater than stock dwell on a distributor machine and watching a scope trace is the best test. If you don't have a dizzy machine handy, and it passes the simple spark test, run it on a car for several days, carrying a spare. If it runs to your satisfaction that way it likely will last another mere 20 years.
Side note: Quickest way to ruin a coil is to make/ break the points with no output lead connected to the distributor or an arc gap or to ground. If the coil doesn't discharge the high voltage externally it breaks down the internal insulation. Result: a coil that produces a very weak spark.
