If you enabled the 'My Feeds' users will have a 'Feeds' item in their 'My Stuff' menu where it lists all available feeds they can subscribe to.
Alternatively, you can insert a snippet of code in your header insert to make the RSS icon appear in the address bar of the user's browser. The code would look something like:
<link rel="alternate" type="application/rss+xml" title="Feed" href="http://www.yourdomain.tld/forums/cache/global_rss##.xml" />
Keep in mind this will only use one feed -- the one you specify by replacing the ##.