I have lots of both PK (self-tapping screws for sheet metal) and
chipboard screws but I don't always know which are which. Is there
any easy way to tell them apart?

I guess anything with a pan-head or a cheese-head is fairly likely to
be a PK screw but countersunk heads present more of a problem.

I suppose I can try actually screwing them into a sheet of aluminium
but before I do that I thought there might be an easier way.

--
Chris Green