[header] How to check an ac compressor [title] Turn off the power leading to your air conditioning unit. [step] It's important that your power is off so that you don't shock yourself when checking the compressor. Many central air units will have a power shutoff switch on the wall next to the unit. 
Find the switch and switch it to the off position. [substeps] If there is no switch, find the circuit breaker that controls the power to the area of the house where the ac unit is.