Re. testing PSU with multimeter :
Unplug everything from the PSU (mobo, drives etc)
Connect the mains cable to PSU & turn on at mains etc.
Because ATX supplies are soft power (i.e. the on/off switch on front of PC connects to motherboard, not the PSU) you will have to force the PSU on.
To do this, identify pin 14 on the main 20 pin power connector of PSU. It is usually green & will be the fourth pin from the end. Use a short bit of wire (or metal paperclip etc) bent into a U shape. Push one end of wire into pin20 on the socket & the other end into one of the black wires on either side. If PSU is not blown, this should force it on.
Assuming it works, connect black probe of multimeter to one of the black wires in the connector & touch the other wires in turn to read the output voltages. Not every wire will produce voltage, but the ones that do should read either +/-3.3V, 5V or +/-12V.
Make sure that multimeter is set to measure DC Voltage (e.g 20V range) and NOT set to current(amps).