[rotorwrench]

Don't forget long strait edges. I also measure from the right front frame horn to the rear left end of the rail then check left to right & compare the readings. This will tell you if it's still square front to back. String lines will get a ball park figure but a person can measure the gaps with a straight edge. On frames with a natural bow built in are best checked on a frame table but where there is a will, there is always a way. Sometimes a person needs a known to be good frame or an original factory drawing to check all the dimensions depending on how complex the frame design is.