To verify your RSS feed is valid, simple go to the following site(s) and insert your feed URL:
Feed Validator - https://www.feedvalidator.org/
W3 Feed Validator - https://validator.w3.org/feed/
Ideally, the result will display that it is a valid RSS feed.
Below are additional services created specifically for podcasters:
CastFeedValidator - http://castfeedvalidator.com/
PodBase - https://podba.se/validate/
There are certain warnings that are acceptable or known issues which will not prevent your feed from being validated or accepted by iTunes.
1) FeedValidator.org does not recognize the latest iTunes specifications such as closed captioning and order attributes. You can safely ignore these notices.