I use rspamd.

This might be a little heavy if you’re solely looking for DMARC validation, but I use the other parts of rspamd as well for inbound email.

https://rspamd.com/modules/dmarc/

For a library, I mainly write Go and I use https://github.com/emersion/go-msgauth (formerly known as go-dkim).

Url seems to be https://docs.rspamd.com/modules/dmarc/