Short pins 4 and 6 with a piece of wire (green and the one two holes to the right). Do it carefully or prepare for unforseen consequences.
If the PSU and plugged-in drives spin up, the supply works.
Also mess around with what auxilliary connectors are plugged into the mobo. There's a 4- or 8-pin CPU plug that you absolutely need. There's a 4-pin molex that sometimes is needed, but sometimes makes it not work.
[edit: Pin numbers 4 and 6 are as counted WITHOUT the ATX+4 attachment.]