You can go to Gmail, open the email from your mail server, click on the three dots menu, "Show Original".

There you can find some hints, e.g.:

  SPF: PASS
  DKIM: PASS
  DMARC: PASS

If any of those are not in 'PASS', you need to fix it and retry.

Yes, setting all of these up is not trivial, but it's also not the monster people usually claim it is and I think we should all do our part that this knowledge doesn't appear so unobtainable, as we're already centralized more of the internet than we should have.