In a rear derailleur the axis of the main pivot joint and upper and lower derailleur wheels should be parallel all times. Derailleur cages often get bent at crashes so that particularly parallelism of upper and lower derailleur wheels are lost leading to such as twisting of chain, etc. (Modern derailleur cages are often very thin and not of steel anymore as was in the 7speed ages). Due to the short distances involved judging by eye sight is not very reliable. Are there any special tricks or gadgets (may be similar as for derailleur hanger)?