I do not deliberately measure traffic. And I certainly never put UTM parameters in URLs as a sibling comment mentioned, because I find them ugly. My personal website is a passion project and I care about its aesthetics, including the aesthetics of its URLs, so I would never add something like UTM parameters to them.

I only occasionally look at the HTTP 'Referer' header in my web server logs and filter them, out of curiosity. That is where I find that a large portion of my daily traffic comes via RSS feeds. For example, if the 'Referer' header indicates that the client landed on my website from, say, <https://www.inoreader.com/>, then that is a good indication that the client found my new post via the RSS feed shown in their feed aggregator account (Inoreader in this example).

Also, if the logs show that a client IP address with the 'User-Agent' header set to something like 'Emacs Elfeed 3.4.2' fetches my '/feed.xml' and then the same client IP address later visits a new post on my website, that is a good indication that the client found my new post in their local feed reader (Elfeed in this example).

For incoming traffic you can use ?from=rss or utm. To measure traffic to rss itself just parse server logs

You add UTM tags to the links, like everyone else.