I'll go out on a limb and say there are many tried and true methods.

How I do it is...
Front droop; lift the front end until the steering blocks are just topped out, measure with ride height gauge behind front wheels, under the chassis edge.
Lightly compress front end, let it rise, and measure again.

Rear droop; lift rear end till center shock tops out, measure with ride height gauge at rear most point on the main chassis edge.
Compress rear suspension, let it rise, and measure again.
The measured difference from fully topped out, to loaded, is the amount of droop.