If your PSU is an AT syle , is is simply a matter of turning on the mains power to it.
If it is an ATX , the you have to grond the PS_ON connection (usually pin 14),then you can monitor the PS_OK line for assertion (active high pin 8 )to ensure your voltages are correct.

The secondary of a PSU is isolated from the primary , but there usually very cheap inside and when you get a problem its usually a big one.