most chickens will get mites or lice. some are affected worst than others. i had a pullet last summer fall off her night perch and she had been so stressed out a couple days before but was still acting slightly normal, just looking a little sluggish. when i brought her in and checked her over i saw them all over her. the other girls were fine in the coop but i have seen an occasional mite or louse on my other girls and they don't seem to bother them enough to stress them out in the same manner.
i would say every month you should dust for lice/mites since that is the time any of the bug's eggs could hatch. you can get stuff called garden/poultry dust , also known as Sevin dust. mixing diatomacious earth (DE) in the shavings or ground litter should help to control them down there-since that is where the birds will most likely pick them up from. also, mixing DE in the area where the birds like to take dust baths is a good idea too. i used 5% carbaryl (sp?) since that is what the feed store had and knew it was safe to use on dogs/cats and in/around the garden (stuff is the equivalent to Sevin). though, with any of these powders you need to wear long sleeves and you should have very good ventilation but not where it is windy since the powder will fly everywhere from being so lightweight.
as for looking for them you would most likely see the mites around the face as little white dots moving about. they are usually active during the later hours of the day/night. the lice are about 1/8" long and tend to 'scrub' the chicken's skin and feathers for dead skin. seems like they should be somewhat beneficial but they stress out the chicken doing that. if you got one on your skin it feel disturbing but they won't live on humans, they specifically feed on chickens, so there is no need to panic.
when i used the Carbaryl, it took my girl about an hour to get most, if not all of the mites and lice off her. i actually had gloved hands helping her groom them off while she was working on them too.
this may help with treatments of the coop and walls.
http://ohioline.osu.edu/b853/b853_11.html
i found this but i'm just a little disturbed by it (and the chicken might be too)! especially since it won't treat the mites that like to get onto the head region of the bird.
http://www.ehow.com/how_5597236_treat-lice-mites-chickens.html
this is what a poultry louse looks like close up. U-G-L-Y!
http://www.agf.gov.bc.ca/cropprot/poultry.htm