Since it's a slab house, it probably won't be too deep. To find it takes
an educated guess. If the house has only one bathroom, or maybe two of them
back to back, the sewer probably runs out roughly in line with the toilet.
Often it lines up with the vent pipe coming through the roof.
The rule of thumb is that the pipe comes in where it meant the minimum
amount of digging. Start digging as close to the house as you can.