A very good SPF checker:
https://vamsoft.com/support/tools/spf-policy-tester
Here is possibly the original SPF checker:
http://www.kitterman.com/spf/validate.html
To do a deep study of a bounceback, the best tool this writer has ever seen is here:
https://testconnectivity.microsoft.com/
in the Message Analyzer tab. Just paste every header you have into there and submit, and then read. Lots and lots of excellent information comes up.