I stick a fork in it and twist - if it twists easy it can be pulled. Never had a problem with the poked holes having the juices run out on the pork butt. I also do like another said, do not trim and put it fat side up.
I usually keep the smoker at 215-225, 225 being the target, and start checking after 6 hours every 1.5 hours.
I recently did 4 shoulders (I normally only do one at a time). I figured that I would just cook until the largest was done and the others would be good (two at 6.5, on at 7 and one at 7.5 pounds). It's hard to over cook the shoulder. Anyway when the largest was done I took them off. Two - a 6.5 and the 7 pounder fell apart taking them off the grill, one of the 6.5 pounders I could barely pull, and the 7.5 pounder was perfect. I pulled the tough one first and thought I had REALLY miscalculated the fork twist in the 7.5 pounder.
So, I never could figure out why. I just assume that the individual piece of meat matters quite a bit in factoring how easy it is to pull. I've noticed some single ones I cooked for a LONG time and never got real easy to pull, others were at the 6 hour mark. The above experience made me decide that it is the piece of meat as it was the only variable in what I did.