What's eating my brassicas overnight?

I planted out a load of cabbage and broccoli seedlings last week and something is decimating them. The damage seems to happen overnight — large ragged holes in the leaves and some plants almost completely stripped. I haven't actually spotted the culprit yet.
I do have netting over them but I think something may have got underneath. Could it be caterpillars already at this time of year, or more likely slugs? Would love to know how to identify the pest before I treat for the wrong thing!