It's a bit windy at the moment, but standing the hen on a fully opened paper feed bag and ruffling through all her feathers sometimes shows up lice or mites. We have picked hens up at night, shortly after they have roosted, and brought them into a cage with kitchen paper on the bottom. In the morning we have lifted the paper and there were hundreds of mite hiding under there. Seems as soon as the birds roost, all the mite come out and descend on them.