OK, the two black leads are the main hot and ground. Ground is the bottom, hot the top.
If the iMac is plugged into power you should always get 12V there whether it's on or off.
When not powered on you should not get any readings at the molex connector at top of PSU or anywhere else inside the case.
The fuse is the black cylinder just above the hot lead.
I doubt it's bad as you are getting readings past the main hot. If it were blown you would get nothing out of the PSU when powered on.
Look to the right of the RAM slots. There are 4 diagnostic LEDS. How and when they light up can help you determine where the problem may be.
Check the Technicians PDF I posted for the sequence.
As far as testing the PSU out of the case you don't really need to, just disconnect the molex at top of PSU and it will be isolated from the rest of the components. Check for readings at the connector.