How to tell number of 12 volt rails

Hardware PSU
1

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

Thanks

2

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. 

3

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