To find out if feed is working ok...
To test wether your feed is updating normally:
1) Create a new post (any post will do). Mark down the title.
2) Access your rss feed and do a search on this title (you can use CTRL+F to initiate a search in the page).
The new title should appear inside the <title> tag, on top of your feed with the appropriate publication date (visible under <pubdate>)
Here is a few thinks to check if your feed is not updating.
Check if your feed is cached
Enabling cache is a recommended method to improve your overall website performance ans SEO. Enabling cache will result the server to store your page in a static content or even on the visitors browser and present visitors with the stored copy of the page (without querying your database). The downside of this method is that this page may not be up to date (yet) with the most recent content. Since RSS feeds are time sensitive, it should not use any Cache.
If you're using a plugin such as W3 Total Cache plugin or any other tool to cache your website pages, make sure to disable cache on your RSS feed page.
Is your feed too large?
If you're publishing a large volume of content, your feed could end up being pretty large and heavy to load. This can cause external services to timeout / fail when accessing your most recent content.
To avoid this, simply restrict the maximum number of posts stored in your feed. If you use wordpress, you can control this from your Admin as shown below. No need to have hundreds of elements in there 10-20 most recent items should be enough for most of us.