+1 to the X2 wireless products -- anything I could have listed in the disadvantages section got erased when they released their XDS95 unit. They really have brought affordable, reliable wireless to the masses. You'll need a little more scratch than $300 to get one new but it's worth every penny.
Actually, you can almost always tell.
I always use an outlet checker before I plug in. Always. I always use GFCI protected quad boxes. I always use a "wiggy" to check for voltages between mics and guitars and what not. Always. The checking and double checking adds a bit of time to set-up, but I figger the shocks I don't get might add decades to my life.
I always use the standard AFM contract which requires the purchaser to provide a safe working environment for the musician and the musician's equipment. Safe environment includes electricity. If the purchaser refuses to sign the contract, that's a clue to me that they really don't want to commit to the price we agreed upon, or to providing me with a safe place to work. My standard line when presenting the contract is that using the contract allows me to participate in the AFM pension plan, (this is 100% true). The pension plan is a good to great one, and it doesn't cost the purchaser anything extra. It's been a long time since a purchaser turned me down on a contract.