How do I know if my kittens are healthy?

They yowl only when they're hungry or lonely, or when they're mum leaves them in a corner after/ in-between feeding. They're 6 days old, they can crawl slightly, but can't walk or stand. They're eyes aren't open. They sleep pretty wee and twitch a lot, they're stomachs are slightly bloated. How do I know if the mother's secreting enough milk?