What is the best way to measure / verify the condition of the taper in rear hubs?
Wild idea - Tape over the small end and pour in some kind of casting material, then pull out the casting and measure?
Try it on a good axle taper and see if the tapers lock. If it seats too far in then see how much shim it takes to place it in the correct position.
For a better answer I would give Les Schubert a call.
When they're really worn = been driven long distance rather loose on the taper, then it's easy to see how much the keyway is worn. It gets wider than a standard key, except for a small unworn area by the end.
If it's a rare hub, like for a wire wheel, it can often still be used with a special made stepped key and a thicker than usual shim. (But the nut may have to be checked for proper torque more often than with a better hub)
I've tried to get better fit between a hub and an axle with valve grinding compound - might even help the taper to lock if you leave a little coarse carborundum paste in there?
I have a really good, maybe new old stock hub that I use as my gauge. I slip it on a given axle shaft till snug, then remove it and slip on the hub in question, noting how much further the unknown hub slips onto the axle.
As others have offered as well, you want to see how well the hub taper aligns with the axle taper. Prussian blue would work well for that.