You could get a guess by looking at the TDP of your processor and the TDP of your video card. Then maybe add 5W for the motherboard, a watt or two for USB peripherals (unless they are 2.5" hds), 12-15W spinup for hds (maybe 5-7W under use, probably a couple less for idle). And a few watts for any other PCI or PCIe cards. Then take that number and add 20-25% to that and thats probably a rough guess of what the max is you will pull from the wall. I have you add the 20-25% because PSUs are not 100% efficient, so they'll draw more out of the wall than your components are asking just because some of the power gets lost in the conversion from AC to DC.
Now most of the time your processor and GPU aren't pulling anywhere near what their max TDP is. So you could probably google around and see similar systems' idle power with (ideally) your GPU and CPU. It probably won't happen ideally though and you'll have to do some deduction to figure out how much each pull on idle.