"As a sort of side note, I've always found slings to be a terribly vexing weapon, as they don't leave much in the way of archaeological remains behind. You have to figure that slinging greatly predates specially-manufactured biconical slingstones, but unmodified stones are impossible to track, and slings are organic, so there's no way to prove just how old. I think one alternative way forward might be to do experimental work and look at expected injury patterns from sling use, in the hopes of finding injuries which are "consistent" with the use of the sling. Alternatively a look at bone plasticity might help, if slinging differs substantially from ordinary throwing in the loads it places on the body. Of course, I'm not sure the latter is true, or that sling wounds are distinguishable from club wounds when you get right down to the damage done to bones."
I've been hitting the same wall trying to do research on this stuff for the last 2 years. I would think that constant slinging would leave a skeletal signature - much like longbow use does - but I doubt that the average shepherd, hunter or warrior would use it enough to produce these signatures. The longbow signature appeared precisely because of the extensive practice and powerful bows used. I imagine there's a similar signature with powerful composite bow users. So while sling specialists like the Balearic auxilliary should have some sort of skeletal modification, I don't think that modification is likely to be a good indicator of lower levels of use.
Similarly, distinguishing slingstone impacts from other blunt trauma could be difficult. Wells (1962), looking at Peruvian burials for trauma evidence, does have a snippit about how many depressed skull fractures are the same size as the sling projectiles, but it seems like he's imagining weapons in general to be more standardized than they really were.
The problem is frustrating enough that I just went into the ethnographic records instead