Any home sewer clean out at a home goes into a sanitary line. Some houses have a storm drain at the curb of the property, but I'de like to believe that all of us would know the difference!
No, not a storm drain - a storm water management system on my property consisting of 2 underground dry-wells and an overflow pipe that runs off into the edge of my front yard. It consists of buried 4" white PVC pipe with three 4" clean-outs over a couple-hundred-foot run. I didn't know any of it was there until I started pulling the caps off of the clean-outs. The plumbing is identical to my sanitary sewer line, including the clean-outs, and also in the same general area.
My point is that a white cap sticking out of the ground may not necessarily be a sanitary sewer clean-out.
Ok, well there is an exception. However I would like to think if someone went through the trouble with that kind of system then they know what they have. Or if they bought the house it was explained to them. Our storm water drains at the curb are marked "drains to river" with a picture of fish on them.
So, do we agree that "a white cap sticking out of the ground may not necessarily be a sanitary sewer clean-out"? I would think that conducting a simple test as suggested by Turbojimmy before discharging the waste water into the drain might not be a bad idea.