Why not simply looking for existing virtual environments?
To check where they are stored you can activate one virtual env and get its path:
$pipenv shell
$pipenv --venv
> /path/to/existing_venvs/current_venv
ls /path/to/existing_venvs/
> cython-pr4GGvfQ/ openCV-f4drK-E5/
I don’t know if pipenv can store venvs in other places, but at least for me this is enough to know how many and which venvs I have.