I like this one from NIST: https://www.had-pilot.com/py/had.html

You just send an email to the address at the bottom and it replies with a bunch of information regarding its SPF, DKIM and/or DMARC.

For something more complete: https://glockapps.com/

This gives you (in the paid version) a "seed list" of emails at various ISPs, so you can see how your email does with gmail vs yahoo. This is one of the several services, but the one we're most happy with.

Cool tool. I'll have to play around with that and see how it goes. May end up adding it to the article :)