With butts the bone is a good indicator. It will usually pull right out clean.
I have had the bone get stuck. (somehow it seems the rub dries a bit on the outside of the meat around the bone & sometimes it sticks to the meat) 1st time it happened I left it on a bit longer than normal & felt like it was a little dry but noone else thought so. I'm not sure theres a fool proof way to tell if a butt is done except to cook a lot of them and after a while you get to know your cooker intimately & you just know when it's done.
S&W 500 Mag Traeger 075 (2) Traeger 070 a Spit a UDS ORANGE Thermapen