http://www.techpowerup.com/articles/other/22

How do I manually turn on an ATX power supply?

To turn on an ATX power supply that isn't connected to a motherboard use a wire or paper clip to short the green wire (PS_ON) to any one of the black wires (COM).

Also, be aware that some of the older Dells (older than Dimension 4300) did use proprietary powersupplies that are not wired the same as the ATX standard, even though they will look physically the same