Most of the experienced technicians I know use good quality DMMs (digital multimeters, see Figure A). If you are unsure or uncomfortable with working around high voltage, don’t ! Shorting one by mistake feels just like being electrocuted by 120 volts from a receptacle. Capacitors can maintain a line voltage charge for a considerable period of time. Even when unplugged, a supply should be considered dangerous.
I do not recommend that any inexperienced person open the cover of a power supply. Note that you must check the output voltages while the supply is under a load, which means that it will be installed and running in the PC. This will show whether the supply is operating correctly and if the supplied voltage is within the proper tolerance range. One of the simplest tests you can perform on a power supply is to measure the output voltages. Circuit breakers popping when the PC is turned on.
This article outlines how to diagnose a faulty supply by testing its voltage outputs, as well as how to replace the defective unit. Experience can help a tech diagnose problems caused by a faulty power supply that would normally be overlooked by a novice. A bad power supply can be the root of many PC problems.