Badgers strip the skin off the lambs, so it's easy to tell if they've been at a one - you get an inside-out skin and some skeleton, maybe part of a limb, left. You can't know if they killed it though - and even if they do, no doubt it'll be a weak, sickly one they kill. I don't think they'd take on a ewe, either, so a healthy lamb being protected by its mum is probably pretty safe.