How to use is_feed() to target a category feed?

If you want to determine if you are in a category feed, for example:

 http://example.com/category/football/feed

you can do so with

add_action('wp', 'mycheck');
function mycheck() {
    if(is_feed() && is_category() ){
        // do stuff    
    }
}

or

add_action('wp', 'mycheck');
function mycheck() {
    global $wp_query;

    if($wp_query->is_feed && $wp_query->is_category){
        // do stuff
    }

}

You can similarly check if you are in a tag feed, for example:

 http://example.com/tag/liverpool/feed

with is_tag.

You can then check if you are in a custom taxonomy feed, for example:

 http://example.com/country/england/feed

with is_tax.