Events for EditorGUILayout.Popup

you can track the change in the OnInspectorUpdate method, it is called 10 times per second.

 int index = 0;
 int newindex = 0;
 string[] jointsList = new string[] { "Fixed Joint", "Character Joint", "Hinge Joint", "Spring Joint" };
 
 void OnGUI()
 {
      index = EditorGUILayout.Popup(index, jointsList);
 }
 private void OnInspectorUpdate()
     {
         if (newindex != index)
         {
             newindex = index;
             OuChagePopup();
         }
     }
 void OuChagePopup()
 {
         // you code
 }