Typically this is done by a hardware review site that modifies the motherboard so that they can measure the amount of power being supplied to the graphic cards and same goes for the CPU.
Techpowerup.com and Xbitlabs.com are examples of sites that measures the wattage.
Here's a peak power chart for the GTX 570 from Techpowerup.
AMD and nVidia tends to "exaggerate" the recommended power supply because:
1. They do not know exactly what components you have in your system.
1a. AMD CPUs tends to use more power than Intel CPUs.
1b. How many hard drives do you have? I have 6 + 2 optical drives. They all require power.
1c. What kind of motherboard do you have? A "budget" mobo has less features so it tends to consume relatively little power (35w) compared to a "deluxe premium" mobo that has many features that can consume much more power (70w - 95w).
2. Do you overclock? This uses more power.
3. Do you have a crappy, average or high quality power supply? I stick with high quality.