If you are cooking at higher temps foiling can be helpful to keep the exterior from overcooking while the interior finishes. If you are cooking >285/290 then foiling during cooking might be worthwhile. I foil briskets, e.g., because I cook them at 325-350. Butts I would rather not foil during cooking so I cook at lower temps. Some will foil butts during cooking even when cooking at lower temps, but this is usually because they want (or need) to speed up the cook. It is effective for that.
Foiled during cooking or not, the butt will feel very tender and soft when it is done enough for pulling. It will usually be ~190-195 when this occurs, but may be higher. Use temp as a guide to determine when to check, then simply feel the meat with a probe. When tender and soft, pull, wrap and rest.