If they aren't being fed they will get wrinkly and look dehydrated. If they are being fed, usually if it hasn't been too long since the feeding, you can see they have big bellies like they swallowed a ping pong, and you can sometimes see a white splotch where the belly full of milk is(unless their fur is too grown in). There's a picture in the link below, that shows a full milk belly.
http://www.thenaturetrail.com/rabbit-breeding/mother-has-no-milk/
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