By Code definition the line from the municipal sewer main or septic tank to the structure is defined as the "House Sewer" while the main drain line running under the structure is defined as the "House Main Drain". The code also requires that each structure must have a main cleanout at the point were the House Sewer and Main Drain meet. For houses built on slab the cleanout is to be approximately 3" outside the footer wall, whereas houses that have basements the main cleanout is immediately inside the footer wall.

Generally the house sewer line is run as straight as possible from the structure to the municipal main or septic tank, therefore you should determine which side of the house your municipal main or septic tank is, then check along the basement floor just inside the wall on that side of the house for a cleanout cap in the floor.

I have offered this information because I beleive every homeowner should know the location of their main cleanout, however there are a number of problems with your idea concerning installing a trap on the main line.

1. While there are some extremely rare instances where local codes permit whole house traps on the main line, as a general rule they are strictly prohibited by code.

2. A trap will not prevent backflow.

3. In order to prevent backflow on a sewer line you would need to install a "Backflow Preventer" but here again, there are some very strict code restriction on when and where they may be installed and in all cases it requires permitted approval of the local governing authority. The unathorized installation of a backflow preventer can result in some very serious fines.