pyproject.toml specifies how to build a project using a builder. You'd have to execute the builder and let it build the project to know if the wheel it generates is pure python or not.

As for setuptools:

There's a class that setuptools relies on to do the actual building of the Python project, called distutils.dist.Distribution which has a function named is_pure():

https://github.com/python/cpython/blob/e42b705188271da108de4...

Which makes sure that it doesn't have any known external modules or c libraries.

So to do detection of whether an sdist is a pure python, you'd have to execute setuptools and setup.py because of course setup.py isn't just configuration, it's full on Python.

So just looking at an sdist statically you can not know if the sdist is a pure python project or not.