Imagine: we were not Mammals. We had never seen a Mammal. We knew nothing of Mammals.
Could we tell, from fossil bones structure, about breasts & lactating?
From fossil infants. Those too small or weak to be motile had be either hand/beak-fed or nursed one way or the other. Since the notion of lactation is unknown, it will be hard to deduce that. Perhaps a similar system, like an ultra-active sweat gland that produces a lot of water or nutricious liquid for infants.
no
That is basically what lactation is, isn’t it? At least, I am pretty sure someone once told me that mammary glands were evolved from sweat glands.
Or was that your point? (If so, it wasn’t very clear.)
I don’t think so. Even with the knowledge of mammaries we don’t know much about when lactation arose, ref. this paper on the topic where the evidence in the summary is given as:
http://pages.usherbrooke.ca/infosbio/PSL705/bibliographie/Oftendal.pdf
We could perhaps deduce that some form of feeding the infants was done, but if the therapsids had died out I can’t see how Homo Reptilius would be able to guess at the far out modified sweat gland theory when it could just as well be done by puking up nutritious goo or making super nutritious saliva.
And this, is one of the reasons that I love the SDMB! 
The fact that there is usually someone that is highly knowledgeable on just about any subject. 
Aren’t breasts unique to humans? They aren’t udders, and I thought they only became enlarged when other animals are lactating. So if you’re just talking about mammary glands some guess might be made about them based on the bone structure of infants, mainly the lack of teeth, but it’s just as good a guess to assume their parents fed them regurgitated food. There would be no indication at all of the existence of human female breasts.
For the sake of honesty: I knew nothing of the subject and just googled for something scholarly to support my WAG.
Shhh! You’re not supposed to tell anyone that unless they know the secret handshake.
Stop telling people about the existence of the secret handshake!!!
There’s a book called “All Yesterdays” about how we may have incorrectly reconstructed fossils by not accounting for soft tissue, and it includes a chapter on how modern animals might be hilariously misinterpreted if we didn’t have living examples.
Perhaps this helps explain the stunning dearth of fossil porn.
There are few parallels to nursing outside of mammals. I’m thinking that our reptillian descendants would have a hard time figuring out what, exactly, we did to keep our infants alive until they were old enough to eat solid foods.
If the conclusion that the newborns are so weak from their skeletal remains, that they must have been feed a high energy liquid produced by parents, there’s pigeon’s milk and penguin’s milk, produced by the crop of those respective birds. But that’s just an different flavor of “something analogous seen before.” I suppose reptilian space archeologists could infer them from civilizations that made art – the stone age Venus sculptures, for example.
Rule 34, my friend. You’re just not looking hard enough.
This is exactly what I presume the OP’s point to be.
Yeah! Before you know it, somebody’s going to mention the goat!
:: clears throat, glares at CannyDan ::
Well at least I didn’t blab about the squid or the barrel!
On a serious note, milk is produced by several different groups of birds – the pigeon group and the penguin group were already mentioned above. But flamingos also produce milk, using glands lining the upper digestive tract. This material is very high in fat, and includes both red and white blood cells. The young feed only on this milk for about two months, during which period the specialized filter feeding modifications used by older individuals develop. Link. This extended period of development while dependent upon nutrition produced exclusively by the parents seems to be the closest analog to mammals.
(Forget I ever mentioned that other stuff.)