If you have one, it should have a cap on it that looks like this photo. It should be between the street and the house along the drain line (obviously). If you have access to the crawl space or basement, you might be able to figure out where the main drain line leaves the house. Follow that line straight out to the street.
It may be buried or just underneath some shrubbery or something. Also, you may not have one, and may have to add it, depending on the age of your house and plumbing.
We are trying to add one, but are unable to find the line outside.
Oh, sorry. Misunderstood your question. I think you have two options. You can call City Hall or RotoRooter. The city should have plat maps for your property that include the main drain lines. The city sewer folks may also be able to help you.
RotoRooter (or a similar company) can also find and scope the line with fancy cameras and tools.
Going to the township is a good idea, but I doubt they will have the exact location as they always seem to be placed a bit different then the plans end up showing.
You will call the 1-800 line in your area before you do any digging into the soil. Not doing so can result in heavy fines if you get caught doing this without the area properly marked. Also the utility company that comes out to mark their lines may also end up marking your drain as well.
But if they do not mark this do you know where the pipe is located inside? Measure off the corner wall of the house to that point on inside then do the same outside and dig, dig, dig, dig, Eventually you will reach it. If you measure down from the top of the foundation to the pipe this will give you some idea of how deep you will need to dig outside.
Get permits if required from the township.