This usually causes cracks in the paint and can be detected by laying a straight edge on the down tube. Next, sight down the fork to determine if the fork blades are straight in the fore and aft plane, and whether their upper straight portion is parallel to head tube. Another simple test is to ride no-hands and see whether the bicycle rides straight. This will show whether the fork is laterally correct.
Tubes bent by a force at midspan are self evident by no longer being straight.
Rear triangle displacement is measured by stretching a string from one dropout over the head tube back to the same place on the opposite dropout. The distance between string and seat tube should be identical for both sides. Also, because the two sides of a frame are seldom identically strong, dropout spacing will most likely not be correct, one side having yielded differently than the other. Such lateral displacements can be manually corrected by laying the frame on its side, placing the foot on the inside of the lower chainstay at the BB and pulling the dropout of the upper side toward the correct position. Monitor position change by measuring dropout spacing.