What is the best testing method for sewer lines?

Hmm, never heard of using a smoke test or peppermint test for a installed sewer line. These are typical tesats for breathing apparatus fit testing, but they depend on the ability of the apparatus user to detect the odor of the smoke/peppermint intruding through the seal. Sewer lines are sometimes in locations that do not allow proximity to smell for a release. All the municipal sewer lines we test are done with static water or air under a standing head of pressure. If there is a loss of head beyond the acceptable limits there is a leak that must be isolated and identified. I guess you could pressurize the smoke and conduct it like pneumatic air test of the lines. Then the smoke being releasd could be used to locate the leak more easily on visual inspections, once you observed an excesssive drop in the pressure head.

what type of line is it? what is the method of jointing and what depth are you laying at... typically the smoke test are used to find leaks in main lines. the smoke being smaller then a water droplet tends to find smaller leaks around ring joints, and truncations into the main spigots better then your water hydrostatic test. remember a small leak could take ages before you notice the decrease in level.