The PSU I have reads 400W on the label. Its a cheap local one and has only 15A on its 12V rails, so I'm sure that its lower than indicated. Is there any way I can measure the real PSU wattage by any software, instrument, or any means. Please do not suggest kill-a-watt meter as its not available in my country.
@Jook is right about the difficulties of testing the capacity of a power supply outside of a lab. It's expensive and potentially dangerous.
For real-time power consumption, if you can't invest in a Kill-A-Watt, you can try building a separated power cord that you can use with an inductive amp meter. These kinds of meters only work when you can measure the hot wire directly. If you put the hot and neutral wires both through the meter, they'll cancel out. (Explicitly measuring the hot line is basically what the K-a-W does.)
If you want to know, what is the max my PSU could provide - I would say NO, there is no such thing, because you would have to actually use this amount of power until your PSU fails.
This would not be a good idea.
There are surely devices to test your PSU, outside your PC.
And you might be able to determine an avagerage powerconsumption, by taking in all power-consumers of your setup. There might be software for this already - at least I saw some website, which basically takes average ratings for typicall consumers and adds them up according to your configuration.