If you are cooking dry ribs, I find that when the rack bends and the crust cracks in a few places the ribs are perfect. This is for a rib with some tooth, and how both restaurants I have worked at would pull them. However with a thicker, meatier trimmed rib sometimes they will not bend and you need to pinch the meat between the bones to see if it is tender.
Spicewine Custom Medium Large (Thank You Sawdustguy)
Primo Oval (and BBQ Guru for when I need a nap)
ECBrinkmann Vertical Smoker w/mod charcoal pan and thermometer (used mainly for storage)
Meco Aussie Walk-About (AKA the princess grill)
Klose 20x30 Smoker (sold)
Thermos Square Grill (abandoned at Giants stadium)
Kenmore/Charbroil Gasser got dragged to the curb