don't laugh, but there's still one standing in my back yard, it's been there for over 100 years (although it hasn't been used in ages).
based on a "live model", so to speak, and other places where i've seen them, outhouses were often located in back yard of the main house, not in the front. for distance, 100 feet or so. no one liked the smell, so they weren't built too close, but no one wanted to...ahem... "run" too far either, lol.
look for possible signs of a well used path leading from the back door of the house to where the facilities may have once stood, perhaps even an old sidewalk leading from the back door to where it might have have been.
when i get my new machine i'll be concentrating on the area from the house to the outhouse as well as where the clothes line used to be. i'm thinking i "gotta" find
something in those areas. also around former flower gardens and the pathways to where other outbuildings or woodsheds may have been.
Pete