I am trying to find the main drain pipe that runs under the basement out to the city line. Can anyone tell me how to go about doing this? Thank you.

The first you must do is locate the outside cleanout. Once located the house drain will be directly behind it in the basement floor. A floor drain should be a few feet to the side of the drain. If you have a 4" stack (drain pipe) coming down from above that should be the vertical section of the main drain. Does that give you enough reference points to work with? Good luck, Tom