I thought I would write out what we have observed with our very limited experience, for us mostly, so we can refer back to it in future years. All ewes are different of course..
2 weeks or earlier – bagging or uddering up – the udder enlarges.
1-2 weeks, the vulva area becomes pink and swollen
2 days before lambing – ewe is wearing distinct ‘fed up with it all’ face, and getting about seems an effort
1 day or less – being a bit withdrawn from the flock, off her food (even ewe nuts!)