The Three Bs, Bones, Bends and Breaks?
Have the bones popped out and the meat pulling away from them?
Does the rack bend easily when picked up in the middle?
And the meat start to break apart when picking up the rack in the middle?
Has the meat sunken down between the bones?
All of the above are good signs that the fats/collagens are rendering down and the meat is now tender and juicy.
You can cook to temp, you can cook to time and you can cook to tender.
The only one guaranteed to make your mouth happy is tender, tender is tender, whether it is a slight tug or FOTB.
The latter two can leave ya pulling your hair out once in awhile.
Some meat just don't do time or temp like the majority, instead of 12hr PP ya get 16hr PP.
YMMV
Are you certain your thermometer is correct?
Don't be afraid to turn up the heat and/or crutch to get past the stall.