Here are the questions, followed by info.
Would it best to call a plumber to find out what this pipe is for sure?
Are there things that will tell us, as homer and marge homeowners, for sure if we really are on municipal sewer or if we have a septic sys or do we need to call the water co (will they really know or care)?

Have a 4" diameter PVC pipe coming straight up from the ground in our front yard. Goes down about 3ft then connects to another pipe. The top (ground-level) edge of the pipe is just above ground-level and barely visible - we just bought the house and neither us nor the inspector noticed!

House was built in 1956. House papers say we are on municipal sewer (not septic).

Have read many online articles, finding that it is either:
1. a clean-out pipe and can be capped off
2. a sewer air vent (not a sewer gas vent) and should be capped, but with a vented cover (this is what we've done for now)

Other info:
When a toilet is flushed, you can look down this pipe and see the toilet waste flowing through the pipe to which it is connected (we poured a cup of cold coffee in the toilet and flushed... :D).

Thank you!