10
Circa 2009, one great answer was the python library and website at http://www.feedvalidator.org/. It was supposed to be very picky and have lots of tests.
The code still exists at https://github.com/rubys/feedvalidator but hasn’t gotten much attention since circa 2008. It could be a good starting point for a newly-updated library. All the tests could make that easier;
there’s a py3 branch with a bit of progress on it.