With mine (four large half corrugated iron tanks), they are mostly
working in the surface layer. I just take it off the top and put that
in the bottom of the next one I am starting.

If you want really concentrated worms, spread the top on a tarpaulin
and just keep taking a little off the top at a time, in daylight
(maybe not full sun) and they will keep burrowing down to get into the
dark. If you work it into a little depression at the bottom - bingo -
worm concentrate.

That is assuming they are composting worms (not earthworms).
Composting worms = top layer . Earth worms = all through.

Toby wrote:

I have two big compost bins to which I added some worms; they did a
great job but now I want to separate the worms from the cast material
and compost in the bins so that I can start again. Any ideas how this
can be done? No, I don't want to pick them out by hand!