How would/can you tell?

All four quarters should be the same size, with no sign of redness or hardness. You want udders that are carried fairly tight (up snugly against the underline of the cow) so they won't sag when the cow is older and in milk. Cows with saggy udders are prone to damaging them by knocking them into things or having them stepped on.

They should look healthy and be equal in size all the way around. Even if the cows are "in milk" they should still be fairly soft and pliable. Avoid cows with udders that feel hot to the touch or seem harder in one quarter vs another.

Hope this helps.

My father taught me to get the fattest and the heaviest. Love your cows. Feed em right. Hope you have the best milk.