I would keep it simple. Find all the mounting points for the bed, cab, and core support. That is the majority of the points used in measuring systems. Take a tape measure and go from center to center of each hole and compare to other side. Do both length down the rail and cross measure like an x. The length doesn't matter, as long as they are the same. Do it for the rear section, middle section, and front section. All mounting points for key caponents are symmetrical. If it's within 1/8 of inch great. It more then 1/4 could be damaged. Also inspect for visual damage, especially front and rear.
What is wrong with current frame?
2005 Silverado 2500, Crew Cab Short Bed. 122,000 miles
MODS: AFE intake, LBZ mouth piece, MBRP cat- less exhaust.