How to tell number of 12 volt rails

How can I tell how many +12v rails a given psu has, without looking up the model of the power supply?


Look at the sticker on the side of the PSU. It should tell you all of the rails and how much power in amps is delivered on each. 

Exactly. If it has multiple 12V rails, you will see multiple instances of the +12V amperage rating.