For first timers the vulva is often a better indicator of lambing than the udder. It will look longer, larger overall and much softer than usual. If you have any more lambs I'd be inclined to routinely give them good quality powdered colostrum as soon as they're born and again two hours later, as the ewe's milk is likely to come in a bit late and may be of poor quality as she hasn't been fed for pregnancy over the last six weeks or so.