Carbon dating rocks can you
So it's really not difficult, in the same way that a specialist may be able to date a ancient Greek ceramic pot just by looking at the art style and shape of it.In many places in France one can find stone tools on the ground. So there the stone tools were taken out of context, and it's more difficult to date each individual stone.
One lucky thing for archeologists of the area around Stonehenge is that the people of the area used deer antler picks to dig. Plus they are tough enough to be salvageable after 3000 years in the ground.
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If one finds a big biface ("hand axe"), he can consider that particular tool is probably Mousterian, and thus grossly between 300,000 to 40,000 years old.
However, if you only find flakes with no particular detail, you can't really conclude anything.