The proper term for a goat's mammary glands are called her "udder". It is not properly called a "milk sack" though is is sometimes referred to as her "bag". It is most polite to say "udder".
When a doe's udder starts to form for the first time, or an already formed udder starts filling with milk, this is called "making her udder". When her udder really starts seriously filling with milk, this is called "bagging up".
What to look for in a Good udder
- Udders need to have good "attachments", that mean how the udder attaches to the body. The udder should be held up high and tight to the body and not hang down and sag. A sagging udder will not get any better and will only get worse. You do not want a sagging udder because an udder that hangs low will not hold up to many years of milking, will be difficult to milk because it hangs lower than the milk pail, and most importantly, risks damage (I have seen a low hanging udder get ripped open from getting caught on a protruding object, leading to the need for a total mastectomy)