It's easy! (provided you have the supplies)
It's also quick-and-dirty, but it works
All you need are:
An old power cable with the leads stripped
A multimeter
Some alligator clips
Balls of steel (optional)
Disclaimer and warning: This has the potential to be very dangerous as it involves powered, exposed wires. Please be careful and treat any wire like a live one. My table is a bit messy but don't be like me; do this in a clear area. You may also want to try wrapping all exposed contacts with electrical tape for safety and peace of mind.
If possible, set up the circuit on an independent circuit breaker/fuse so you can disable the wall power while assembling. If you are unsure of your abilities, do not try this at home. I nor these instructions are to be held liable for personal injury or damage to your multimeter or mining rig. If safety precautions and common sense are followed, this method will provide a quick reading of your unit's power consumption.
1. Attach one alligator clip to one lead of the disembodied power cable
2. Attach clips to your multimeter leads
3. Turn off miner
4. We are testing for the current (Amperes) the unit draws on the mains so the multimeter must be in series in the circuit we're making
5. Clip one multimeter lead on to the exposed cable wire and clip the other on to one of the PSU's prongs
6. Clip the alligator on the other cable on to the other prong
6a.
Ensure no unintended contact is made between clips, wires, or leads.7. Set your multimeter to AC Amps (provided it can measure ≥10 A)
8. Plug in the old power cable and turn on your mining machine
9. Let the gears turn for a bit and read the Amperage. Multiply your Amps by your Volts to get Watts. For me, 4.85 * ~116 ≈ 562.6 W. I like that estimate because it includes some overhead. For a more accurate calculation, multiply the Watts by your PSU's efficiency (90% in my case): 526.6 * .9 = 473.94 W
10. Buy a new card (optional)
https://i.imgur.com/zXlTLuf.jpg