Some Thoughts on Testing Capacitors

This applies to both fixed and electrolytics.

  A capacitor tester  which does not apply full, rated voltage to a capacitor is, IMO, pretty useless.
  The exception is ESR testers for electrolytics.

Over the years I found that fixed and 'lytics need to be tested up to full rated voltage. 

 An example of this is old paper capacitors may show just fine on leakage at the voltage a DMM applies to them, but when you test them on (for example) an eye type tester with adjustable test voltage; they may show good at 100 VDC applied but when you get up to 350V or more they show short. (Example 600V capacitors) That is the insulation breaking down.

 Likewise 'lytics may break down at higher voltages than a DMM applies to them, or may need to be reformed at higher voltages to their max rated voltages.

I know of some techs that hook a capacitor to a low, fixed voltage ( 3, 6, 9. or 12 VDC) and look at the leakage at that voltage.
 They may show just fine at that voltage, but at their operating voltage they may work for a while, or just fail immediately.

 Also one should also check for capacity to verify it is still in tolerance.

An open capacitor may show absolutely no leakage, but no capacity because it is totally open. 

OTOH, I have tested NOS paper mica capacitors with a Sencore LCR tester at full rated voltage, an eye type tester, and a DMM and the mica capacitors tested perfect. In operation, they had some sort of internal failure and replacing them with new, epoxy coated silver mica capacitors solved the problem. 
  I worked with another tech who experienced the exact same type of problem also, so I am not alone in experiencing this.
 My problem was in a Horiz osc. ckt. and his was in a TV tuner. Again, new epoxy dipped silver mica caps solved the issue. (It cost me 2 days work to figure the problem out, and he spent a week on the set he was working on.)

The moral of the above is test equipment is very useful, but you need to use your brain also. 
Capacitor testers only test under a specific set of parameters, and like tube testers, can show parts to be good when they are not.