No software. Use a multimeter for functions like that. If you want to calculate it using math equations then it goes as follows:
Voltage*Amperage = Wattage. So look at your rails. The +3.3, +5 and +12v each have a max amperage. Multiply the voltage by the amperage to get the total watts for that line, then add up all the lines together and you'll have the total of your PSU. Don't forget to do the same with the negative rails that are listed on the PSU for the complete total