My own rules of thumb are:
Experienced ewes - generally assume everything is alright unless something is clearly amiss. Of course I still check for plump bellies, and look at the ewe's undercarriage in case of misshapen bag / overlarge teats, etc, when the lambs may need help to find their way on.
First-time ewes - never assume the lamb(s) has/have suckled unless you either see them
with the teat in their mouths (it's not enough that they are sucking under there somewhere - they'll suck a piece of wool for ages, waggling their silly little tails apparently happily

) or see that they have plump bellies / aren't hollow.
A first-timer with twins, I think I would always pen for a while if I could. Best, if conditions allow, to let her get them birthed and licked, ideally on their feet, before intervening, but then pen them up so she bonds with both. A first-timer with a single that's clearly suckled, and she's mothering it, I'd leave 'em to it unless there's any reason to interfere.
A quiet lamb is generally a happy lamb - but not always. To check, get it up on its feet - it should stretch, and not look hollow.
A hungry lamb generally cries - but not always. Check the crying lamb's belly for plumpness, its ears and inside its mouth for warmth. If ears or especially inside the mouth are cold, take action.
A newly-born hunched-up lamb always needs keeping an eye on / investigating.
In bad weather / poor conditions my preference would be to have all first-timer indoors with their lambs so I can be sure they are mothered up and suckling properly - lambs can founder horribly quickly in cold wet weather. In good years with good grass and warm weather, the singles will probably all do just fine outside.
There's also a large component of knowing the breed / breeding and likely behaviour - but that comes with experience of lambing and of your own sheep.
Don't forget to enjoy it.
