Not to blatantly repeat all the advice you've already gotten, but I guess I kinda am.
I always brine my pork butts, then cook to somewhere around 160-170, when I get a noticeable stall. Then wrap tight in foil. After the wrap, I will not open the smoker again until 203 degrees, at which point I begin probing. As a few of these guys have already said, you are looking for the butter feel, practically no resistance. This is the point where you stop looking at your thermometer altogether, stick that probe in there and imagine the feel of the meats consistency in regard to shredding.
I have done a handful of butts without the wrap, and had success that way too. At that point you are controlling the bark and the time, but you can get very similar results in the meat either way.
Also, it is very important to probe in several locations, like
thirdeye
pointed out. You may get a different feel in certain spots, so don't be shy.
Finally, the rest is a critical step too. I'm sure someone here could give you a very scientific explanation on why that is, but I basically chock it up to the fact that the carryover will allow the rest of the collagens to break down properly, and then the slow cooling (versus unwrapping and attacking right away) allows all that fat and goodness to really permeate throughout the entirety of the meat.
Welcome to the forums, and please keep trying! Nothing better than BBQ!
Side note: I just put a 10lb butt in a brine tonight, I'll try to remember to take some pics and post it. Wife's bday barbecue this weekend.