As I recall, the old AT power supplies just turn on - Pin 1 (usually orange) is the "Power good" signal (+5v) for the mobo to tell it that all voltages are nominal and it can safely start. There's usually a delay of 0.1-0.5sec before it goes high.
ATX PS's have a line (Pin 14) on the mobo power connector called POWER ON, usually colored green (see Table 2). If you short this to the next wire (Pins 13 or 15, GND, colored black), the PS should start. The mobo usually controls this.
Just remember that switching power supplies need to see a load - either they won't start/shut down or burn up if you try to run 'em no-load! From my book*, usually a minimum load of 0-0.3amps at 3.3v, 2-4amps at 5v, 0.5-1.0amp at 12v is acceptable.
*Scott Mueller's "Upgrading and Repairing PCs" -
http://www.quepublishing.com/promoti...romo=1626&rl=1