I agree w/Rafter. Watch her closely. Is she at all dilated or swollen? You'll usually see mucus (we call them stringers) either hanging or smeared on her hips/rump. Tail to the side or whipping around. With heifers, the teats don't usually fill up as much - they may still look a little wrinkly when she calves. Keep an eye out for changes in behavior; more alert, looking for a place to calve, etc.