How to find the main sewer line?

I have a house that was built in the '60s and would like to have a new sewer line tied into the main for my washing machine. There is a cap on the carport wall for access into the line but I can't find one in the yard. Will there be a map in the county records or something? As an aside, it took the water guy 3 hours to find the water main b/c it was around the corner and down the street (once the original property but not anymore). If it makes a difference, the house is on a slab.