How can I tell if an email is passing SPF?
The best and simplest way to see if an email is passing SPF for your domain is to check a certain detail in the full email header.
You will need to look in the full email header to where the Authentication Results section is and to check if the visible header.from is aligned with the smtp.mailfrom (return path/envelope from). In your case, they both need to be your domain (the domain the email in sent on behalf of).
If those 2 are aligned, it means that email is SPF aligned and it will pass DMARC with SPF.
1. If an email is sent with a From address of email@example.com and the Return Path is mail.example.com. In this case, the email is considered to be SPF aligned.
2. If an email is sent with a From address of firstname.lastname@example.org and the Return Path is mail.acme.com. In this case, the email is not SPF aligned.