Proof load - the correct way is to use a load cell. Look up your local crane company - they have to recertify annually(? - Ron...) and when ever doing repair work so you may be able to stop by or ask at a decent rigging supply Co - they may be able to rent one out for the day.
As far as opposing force - back up between two big trees, strap the rear to one and winch off of the other with the cell at the tree strap - you will need at lease one other person to read the cell as you increase the pull from the truck. But don't dump the clutch or you'll break the shear pin - all you need do is feather the clutch slowly 'till the load you want is reached then stop.
Alternatively, you can use a snatch block up a tree and down to something you know the weight of - ie., front of another truck knowing its FAW (front axle weight) - if that is enough.
Recovery manuals do give factors for ground type, grade, amount of immobility etc., so you could go get something stuck then calculate the factors to come up with the load you for winching - but these are guides and not precise.
As far as percent - I'd go to the load that the winch is rated for or all the way to the correct shear pin breaking if you have a spare. Then you know what you have.
Hope this helps.