With ribs i usually go by time and how they look. They should pull off the bone 1/4" or more when they are cooked. With baby backs I cook them for 2 hours in smoke, 1.5 hours wrapped in foil, then 1 hour out of foil to crisp them up again @ 230º +/-. When I unwrap them from the foil they usually are done. Then I baste them in the sauce of choice for the last hour.