The best way to tell if your field is healthy is to uncover the distribution box and see of teh water levels in it are above the laterals. There should be about one inch of water in the laterals, since the holes are at about 4 and 8 o'clock. If water continuously stands above this level, the the field is slow in absorbing water and should be looked after. The laterals should be at least 12-15 inches below the surface. They should be at about the same level as the distribution box. You should be able to till the first six inches of the top soil without damaging the drainfield.