1) Verify with a polarity checker that the AC outlet has a ground connection and that the hot and neutral have the correct wiring Travel with a long extension cable of sufficient gauge to handle the current requirements as nearby stage outlets will often be unacceptable.
2) Determine what other equipment is sharing the AC circuit with the audio equipment. Lighting dimmer packs, motorized equipment and HVAC units are examples of electrically noisy devices that can degrade the quality of, or inject noise into the audio system.
3) Verify what the maximum available current is on the circuit. This is particularly important when you are sharing a circuit with high current equipment (such as HVAC) and you are using a large subwoofer system that is pulling large amounts of current through high power amplifiers.