She may be restless, not be interested in feed, paw the ground, go and find a quiet corner, maybe under a hayrack if she's in the shed. Watch for the waterbag (don't always see it and it doesn't always appear before the lamb). She may squat but not urinate, or lie down and extend a front leg, push hard, then get up again. All these mean lambing is imminent. If no lamb has appeared within 20 minutes of either of the last two I'd call on a more experienced shepherd to investigate in case the lambs are poorly presented or tangled up or she could have ringwomb.