Scope: Steel frame bikes with integrated deraileur hangers.

Relevance: Relevant on steel frame bikes. On our bikes, we've never bent one, but if you were buying a bike, it could already be bent.

Park Tool DAG-3 or DAG-2.2

 - he could use his Park Tool uses an old wheel axle, 10mm to thread into the 10mm hole where the deraileur is mounted

  ChatGTP: The alignment tool:
 - Threads into the derailleur hanger where the derailleur normally mounts. - - Has a long arm with an adjustable indicator that reaches to the wheel rim.
 - You rotate the tool to several positions (12, 3, 6, 9 o’clock).
 - You compare the distance to the rim at each point. If the distance changes, the hanger is bent.