well it depends on how many rails you have. 2 rails means presumably 12v1 is in the ATX connector and 12v2 is the molexes, SATA power
and the pci-e connectors (if any). 3 rails would have 12v1 in the ATX connector, 12v2 on the molexes and SATA power and 12v3 on the pci-e connectors. you should probably be able to find out which each rail is from your psu manufacturer and hence determine how to test each rail.
in any case 12v1 remains a dedicated rail for the ATX connector.