You need two items to test a capsule to see if it at least has a chance of working:
1. Capacitance meter, needs to read from around 15 to 150 pF.
2. Adjustable DC power supply, capable of generating about 120 volts but almost no current.
3. A 100 to 1000 Megohm resistor to feed power to the capsule from the power supply.
4. A .01uF capacitor to isolate the DC power from the capacitance meter.
Connect test rig to capsule with zero volts. Measure capacitance. You need to know the typical capacitance of the capsule. This checks for at least contact between the elements of the mic. Neumann schematics actually specify this.
Now, slowly ramp up the voltage, and watch the capacitance. You should hit around 65 volts with a nice slow rise in capacitance value, and the diaphragm should not get sucked into the backplate. This checks for diaphragm tension.
While you are doing this, you can watch the gold in the light, and watch for even deflection. If this is uneven, the diaphragm tension is uneven and this can result in a funny sound. This is very difficult to judge, in my opinion.
You can continue increasing the voltage on a normal 6u mylar diaphragm until it sticks to the backplate. This voltage is another indication of diaphragm stretch. I don't know about the thin diaphragms and whether this is a good idea.
Keep in mind that this will not check if the capsule actually produces a decent sound, or for that matter, any sound at all. But it does check that the diaphragm is attached electrically, and there are no shorts, and that there is enough diaphragm tension left to not have the diaphragm firmly sucked into the backplate while you are trying to get it to capture noises.
I have a cheap Chinese LDC that I use to test capsules for sound - some kind of Apex mic that goes for maybe $80.
-Dale