Mostly with an ohms meter. I check the pickups to make sure that each coil gives a resistance reading that it should. I test all the pots to make sure that the total resistance is right. I also check them at "5" to confirm that the taper is audio or linear (I don't trust what is stamped on the case or wafer) and that they all show about the same reading at "5." Since pots of, say 500K, can vary by over 100K, I also check to see that they are at least close to each other. I don't usually do this with new pots, but I have occasionally run a signal through them and listened for scratchiness.
I test all the switches to make sure they are working properly with the ohms meter. In this case I am just testing for continuity...that connections are open or closed as they are supposed to be (1=not conneted, ~0= continuity).
If I use any simple resistors, I test them. I did not used to have a meter that would test caps, but I do now, so I test them as well. I admit that I do this not so much to see if the resister is faulty, but more to make sure I have the right one ;-)
Once everything is connected, I clip a lead onto the ring of the jack and test to see that all the pots, switches, shielding and the strings show as continuous.
On my last project and on the one I am working on now, I have assembled the wiring harness on a template. Once the pickups are installed, I can test the wiring harness before putting it in the bass. This was VERY useful on the last project, an ES style bass. On these you have to shove the whole wiring harness through the f-hole, so you want to get it right. As I was testing the tone control, I realized they were basically not rolling off any treble. I got some advice from Weirdbits about changing my cap values, and was able to do that before putting the harness in the bass. Also, while I was testing on that bass, I pulled the shaft out of one of the pots...and was happy to have that happen during testing rather than when in the bass!
I am testing paint and finishes right now... The more experience I get the more I test everything I can.
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