Like so many moments in Dnd the effectiveness comes down to your dm.
-RAI sentinel seems to mean you get an attack before the attack roll on an adjacent friendly, RAW the attack is after.
-If your dm rules your sentinel attack is to "protect" friendlies and goes before enemy attack roll...
I would say find a excel/google sheet of all the monsters (google it there are like 5 working ones) they usually have a type column that you could just go through and change case by case.
That is the practical answer w.r.t. the other answers given.