some power supplies have a cable you could plug into your computer some how and it mesures how much power its using, and how much it could be using..... ive only heard about it, yet to see it
Yes, but more important than that, is that the PSU outputs stable voltages. I reckon that brand name is more imporant than watts. An OCZ ModStream PSU should be fine for most rigs.
Those things aren't as accurate as they could be. But u do get to the right direction when using that kind of calculations, so u are definately safe with ur power supply.