Forget any thought that they would have used a certain pitch, or any 'standard' installation. They just make sure it has slope. For one thing, the pitch is 1/4" per foot, and some areas allow 1/8" per foot on 4" or larger pipe. That means minimum pitch 1.25 or 2.5 inch per 10 feet. They may exit the house shallow, proceed at minimum pitch towards the street, then drop near-vertical if necessary to the main, which is probably at least 10' deep and in the middle of the road. Or they may have started deeper at the house and used a trench with a modest slope from there to the main. Or whatever!
A plumber with a camera and a locator may be able to narrow it down for you.