I would use a metal probe like we use for finding pipelines, old graves, etc. A 1/4" steel rod about 4' long with a handle welded on one end and a ball bearing welded on the other works real slick.
Just probe around, the leach field will probe fairly easy (almost like butter in some places). Depending upon the soil type and how deep the field is, the first foot or so may be hard but then it will be easy sailing (don't fall over when you hit a soft spot doing it) down through the the leach field (and for that matter, old rotten wood coffins). Untouched dry ground will be fairly hard probing all the way. Use a steady hard push. Watch out though, going down into hard dry clay, stopping, and then starting with a little force will thump like you are hitting something solid and can fool you into thinking you hit something solid.
You could also try witching if you don't keep a probe handy(doesn't everybody?). If your leach field is wet enough and has some pipe, it may work. A couple brazing rods works best (you could try wire or coat hangers) bent in an L shape with about 6" in your hands and the rest of the rod sticking straight out. Hold your hands belly high slightly out from your belly with the rods sticking straight out and loosely held in your hands. Start walking from an area obviously outside of the leach field and into the field. When you cross something metallic (pipe) or possibly the leach field, the rods will cross, then mark the spot. For finding pipelines, I generally walk perpendicular to the suspected location and do this in a number of places and from different angles and sides. Usually the line shows up. It doesn't work so good parallel or on top of a pipe.
Good luck and post back with what you do and if it works. Some people can witch for water and oil but not me.