The well-fed kid is your baseline
In a large litter — triplets, quads — the doe's milk is a contested resource, and the first skill is knowing what a kid that is winning looks like. A well-fed kid has a rounded, springy belly after a feed, sleeps in a heap with its siblings between feeds, wags its tail as it suckles, and gains steadily day by day. Its mouth feels warm when it mouths your finger and it has the energy to bounce. Hold that picture in mind, because the kid that is losing out departs from it in small ways first.
The early signs are quiet ones
The first sign is usually shape: flanks that look tucked and hollow instead of full, and a back that feels bony under your palm. Behaviour follows — a hungry kid cries more than its siblings, follows the doe pestering her between feeds, stands hunched with its head low rather than playing, and feels cool around the mouth and ears because an empty stomach does not hold body heat. At the udder it may butt hard and give up quickly, or get pushed off by stronger siblings before it has drunk. None of these alone proves anything; two or three together, in the same kid, on the same day, are a pattern.
Weigh — do not guess
Eyes deceive, especially with fluffy kids in a pen of near-identical siblings. Weighing the whole litter every day or two at first, on the same scales and at the same point in the day, turns a hunch into a fact: a kid holding its weight while the litter gains, or losing any weight at all, is not getting its share, whatever it looks like. Mark the smallest with a dab of stock marker so you can find it instantly at every check — in a big litter, the kid you cannot identify is the kid you stop seeing.
Check the doe as well as the kid
Sometimes the kid is fine and the supply is not. Feel the udder: one side much fuller than the other can mean the kids are only drinking from one teat, and an udder that is hot, hard or painful, or a doe that kicks kids off and will not stand, points to a problem with her that needs the vet's eye. Also count teats against kids and watch whether the smallest ever gets an uninterrupted drink — some does simply cannot raise quads unaided, and that is a management fact, not a failure.
Act early, and know the emergency line
If a kid is merely behind, the fix is usually help: hold it to the teat while the others sleep, or give a supplementary bottle of the doe's milk or suitable replacer under your vet's guidance, little and often. But a kid that is cold, limp, too weak to suck, or crying weakly without the strength to stand is no longer a feeding problem to manage at home — warm it gently and call the vet promptly, because kids that small go downhill in hours. Regular weights recorded for each kid in KinsLe's Pet Passport are what let you catch the slow slide early, while help is still simple.