This looks like a pretty good resource.

I'd like to add the following, which I continually reference and has led to repeatable success:

https://www.linuxbabe.com/mail-server/setting-up-dkim-and-sp...

And as far as confirming it works, I continually rely on sending to a gmail address.

Under the three dots is "view original" which gives you

SPF, DKIM, and DMARC results.

(edit:formatting)