There are power supply calculators online that let you configure a system and average load, and they in turn tell you what size power supply you would need to run those components at the given load. Granted, that will only get you ballpark, but it's a start.
Many hardware review sites also test power draw on the stuff they review. See anandtech.com and silentpcreview.com for some examples.
If your servers have 650W and 450W psu, then they are almost certainly drawing something less than that. In fact, most systems will typically draw half or less of the rated max from their power supply. On the flip side, the rating is DC output, so innefficiency will dictate that the psu is drawing something more from the wall than its output.
In the end, your best bet is to meter it at the wall, as Jim said.