It's been on low for the past two hours and I don't have a working meat thermometer.
I would pierce the meat with a fork to test the tenderness of the meat. If the meat flakes or pulls apart easily, it is done and you can stop cooking. If the meat is still a tough, keep cooking on low until it is tender but still moist. The worse thing that can happen is that the meat will overcook and be dry and stringy. Because slow cookers behave differently, even among the same models, there isn't an exact time or temperature to use when cooking anything. You have to watch and make sure that the food doesn't overcook. If your slow cooker runs hot, I would think that the pork butt is safely cooked by now and hopefully not dried out.
Thanks that helps - I didn't think of checking it! :)
I think Dave On The grill is the one who said 185f is usually when pork becomes pulled pork tender. Maybe take its temp and also see how it feels and looks like halfpint suggested.
Oops...just noticed you have no working thermometer.
It should be done with 3 hours on high.