"klong"
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.