The most effective technique, one that can save you hundreds of dollars of erroneous digging, is to use a "sonar probe".
This gadget is often combined with a video camera designed to look inside sewer pipes.
An emitter probe is snaked into the drain line. You walk above with the detector, which will give you a lot of important information on its read-out screen. It will tell you exactly where the probe is. It will tell you exactly how deep the probe is. It will also tell you which direction the probe is heading. You can simply paint a trace of the line right on the basement floor or the yard grass, complete with depth indications.
If you use the video camera too, then you even know where the "problem" is and if you have just run past any branches in the line.
Thanks to my buddy Phil from Phil Groves Sewer Services in Hamilton, Ontario for bringing his high tech equipment into studio. You can locate this kind of equipment by looking up "sewage" in the Yellow Pages and look for advertisements that mention video inspections.