Your Datum for the chassis should be the rear axle.
Normally the rear axle should be straight and level.
Therefore the vertical distance from the underside of the rear axle to the longitudinal chassis members must be the same each side.
Hang bobs from the rear axle by the bearings and from the front yokes. Measure the diagonals (left rear, right front) They should be the same.
Put a straight edge across the chassis bars at a known point. Use either an electronic angle finder or an engineers precision level. The straight edge should be in the same plane as the rear axle.
It's worth checking the chassis both with the bits loosened off and tightened to working tightness. It is not unknown for tightening a bolt to tension and put a bend in the chassis.
If you have an electronic angle finder or precision protractor, run a straight bar through the kingpin cassettes. With the same settings each side, the caster and camber angles should be identical. The chance of having the chassis straight and the yokes bent but the angles the same is very small
It doesn't really matter if the longitudinal was meant to be horizontal or not. You are referring all the measurements back to the datum, not to the outside world, so that if the rear axle reads "0" and the bobs are hanging at the same angle (thanks, gravity) then things are straight relative to the datum.
Getting the chassis to bend back into shape is a different art. It helps to have a bending jig at that point.
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