Must have:
1) Power outlet tester
2) Power strip with built-in Ground Fault Interrupter Circuit (GFIC) breaker(s)
3) Multi-meter
Use item 1 to determine whether the power outlet into which you will plug your equipment is correctly wired (all three wires present, none of them interchanged). If the outlet is incorrectly wired and cannot be repaired before the gig, go home.
If the outlet is correctly wired, assume it is either not on a GFIC, or that the venue's GFIC device is broken or defective, and plug into said outlet your own GFIC equipped outlet strip (item 2). Test your GFIC, then reset it. Note that testing your GFIC may also trip a functioning upstream GFIC, if present, in which case both will require resetting.
If the PA belongs to your band, all of the above also applies to the PA. If the PA belongs to the venue, you have very limited ability to force those safety measures upon them.
After all your bass gear are plugged into power and interconnected, and before touching any other gear, use the multi-meter (item 3) to measure the voltage between your bass' ground and:
- microphone
- mic stand
- any other potential source of lethal voltage that you may touch during the gig