Modules/getpath.c seems to be where the C code does it when getting paths for sys.path. So it would be possible to use that same algorithm to set some sys attribute (e.g. in_checkout or something) much like sys.gettotalrefcount is optional and only shown when built with --with-pydebug. Otherwise some directory structure check could be done (e.g. find importlib/_bootstrap.py off of sys.path, and then see if ../Modules/Setup or something also exists that would never show up in an installed CPython).