There are so may variables in the power consumption of PCs that any kind of simulation to calculate the power consumed by a particular m/b + chipset + cpu + graphics adapter + all the other bits would, I think, be very difficult.
As for the power that a psu can provide, the true test of this is really a hardware based test - load the PSU up to its rated capacity on each voltage supply rail and see how the voltage regulation, ripple, noise etc. suffers (or not). This is really a lab based exercise needing variable loads and decent measuring equipment, and not the kind of thing that the average computer user could simulate easily.
Some of the lower end of the market/less well known PSU manufacturers can be over optimistic in declaring the max. rated output of their supplies. If in doubt, choose a well known brand name and err on the high side for power rating.
In my spare time I fix/update PCs for friends and work colleagues and I have noticed that some of the PSUs in systems over a couple of years old seem to be very much rated on the low side and any upgrade to a better m/b + cpu usually necessitates A psu upgrade as well.