You'll hear different "distress" calls over time and know to recognize them.
When mom is cleaning/licking their bums, they let out a unique sound. Once they start going on their own, you might hear the same sound once in awhile. The licking seems to encourage "pooping" and eventually urinating. She might pin them between her front legs (i.e. an ambush cleaning) or she'll do it when they are nursing. You'll see them sticking their bums in the air and flaring their sack hole like boars do as adults as well. If you are hand-raising parent-less pups, you have to encourage this yourself so they'll go if I understand it correctly.
Corrective "nips" from mom, need to be left alone as the pup needs to understand what she's teaching it (manners). You'll hear another kind of distinctive stress call, which reminds me of a sulking child that just got scolded by a parent, after she nips. Eventually, you'll see them look away and seemingly "pretend" to be interested in something else immediately after a nip, or they'll freeze up and stare in another direction while making the "sulking" sound. It's 100% normal. It'll happen on and off over the weeks, and once the pups are old enough, they'll nip each other once in awhile as they establish boundaries and presumably hierarchy status, though interestingly enough (mine) don't sulk from each other.
From what I can tell, my sow started the nips kinda early, as if to "prime" them so they'd understand what the nip meant. If they stared head-on at her while getting too close, that seemed to be a trigger where she'd correct them. Other times, if they got over enthusiastic and where bum-rushing her, she might nip in succession (I have 4 pups and 3 were almost always chasing her non-stop) so it was like a boom, boom, boom and she just corrected 3 of them in less than a second. They'd sulk for 10 seconds, and back to chasing mom or they'd leave her be.