Get the frame setting as level as you can on the driveway, garage floor or on adjustable jack stands. Next you will need a piece of string and a pointed weight. Start at each corner with the string held up to a given point on the frame and let the weight just skim the ground. Mark the point on the ground with a marker of some sort. Go front to back on each side marking key points all along the frame and suspension.
Now take your tape measure and measure from point to point on the ground, you will get a much better idea if its off since you can hold the tape level.
Joe
