About the only "quick" way I can think of that would even be potentially accurate enough to do yourself is to raise the vehicle all the way off the ground so no susension components were being used to support the vehicle weight. Vehicle and floor would have to be perfectly parallel.
Any measurements on the ground are pointless, springs, more weight one side to the other, and tires (along with a few other more uncommon things) can all have an affect on "height" when being used to check for frame straightness.
Not to mention, there are very few "flat" parts of the frame...GM used various holes punched/cut/drilled through the frame to check for frame straightness.
