I'm no expert but:
I think the white coating is very much older than humanity and therefore you should be looking at the darker, fresher surfaces.
The 'giveaway' evidence for humans working a flint is a 'conchoidal' fracture:
Conchoidal fracture - Wikipedia, the free encyclopedia
Just the one surface showing a conchoidal fracture could be the result of a natural accident. If there is more than one, evidence for a worked flint starts to build up.
Some flints are found that look like they've been worked to a useful shape, but the fracture planes show no evidence of a conchoidal fracture. The fractures are the result of frost damage, and I understand are referred to as 'starch', apparently because of their similarity to starch grains.
Jim