Very good question.
The answer lies not in the remains of the tool themselves, but in the analysis of the debitage. Tools are rarely found, and hammerstones are no exception. For example, the Gault site in Texas (one of the largest, undisturbed, clovis workshops ever found) contains only ONE hard hammerstone. No pressure flakers, no billets, no soft hammerstones, nothing but the one hard hammerstone.
The evidence of what the workers used for tools lies in the debitage: "Although not found, osseous billets were likely used, because much of the debitage exhibits platform lipping and bulbs of force are diffuse." Clovis Lithic Technology, Investigation of a Stratified Workshop at the Gault Site, Texas: Waters, Pevney, and Carlson, 2011, page 36.
(osseous means bone and/or antler)
So, aided by the process of "replication", archaeologists are able to look at the actual debitage, then compare that to debitage made by modern knappers under observation, and surmise that billets were used.
However, the reason this is such a good question is the fact that archaeologists together with modern knappers make a guess as to what tools were used and then, when they are able to produce similar results as the ancient knappers, they conclude that they have discovered the method, when all they have succeeded in doing is finding a possible method. And many times the knappers are inexperienced, exposed to only one or two methods, and are often the archaeologists themselves. Not good!
That's why I make such a big deal about why I think only expert knappers should be involved in replication. The best knappers are familiar with MANY techniques, tools, and debitage types and are, hopefully, less likely to fudge the results to make it seem like their time replicating has been productive!
OK, that's my spiel for the day.
