>> I'd be curious to know how they determine that they're tools and not 
>> just
>> rocks.

> Tresources/multimedia/videos/how-tell-rock-stone-tool

In case the video isn't explicit enough, the clearest indication is the
context in which they are found.  Our ancestors kept really messy 
"shops"
and would make a number of tools at a time.  So the mess they left 
behind
is one context that indicates they are tools rather than broken rocks.

Another clue is that tools were made using particular strokes, and 
consistent
tool making strokes aren't replicated by natural causes.

A tool found without any context will be difficult to identify as a tool 
in
many, perhaps most, cases unless the shaping of the edges shows 
deliberate
strokes or can be well compared to another tool with known provenance.

Mike in Sacto