As part of every preflight grab the tip of the fin rear spar and the end of the horiz stab rear spar and gently pull them towards each other. There will be a little bit of flex but not much. If a change is observed during a preflight and there is noticeably more give between the fin and the horiz stabilizer, that is an indication that the transverse bulkhead between the last 2 bulkhead stations is cracked. This small transverse bulkhead is a major attach point for the vertical fin.