I think it's a bad on/off switch, how can I tell?

My computer won't restart after a local power failure last night. Our
neignborhood lost power for 7 hours, but the computer was on a UPS
when we lost power. I shut it down with the power switch , but after
power was restored, it wouldn't start. I thought perhaps the PS was
fried in spite of the UPS because when the power went out, it didn't
go out all at once, it "flickered, that is, on and off rapidly for
several seconds. Anyway, I installed a old PS that I had used in
another computer, but no effect. Now I'm suspecting the on/off switch
on the case. There are 4 wires from the switch to a pin set on the MB,
red, black, white, and green. Can someone tell me if it's possible for
me to short two (?) of these wires to bypass the on/off switch, and
thus see if indeed it's just a bad switch? And it that's the case, a
way to simply bypass the case switch and get back in operation?