Conditionally prepend custom text to post titles in WordPress. Source: http://davidwalsh.name/prepend-append-wordpress-titles

prepend-custom-text-post-titles.php

<?php

//* Conditionally prepend custom text to post titles in WordPress
add_filter('the_title', 'customprefix_prepend_text', 10, 2);
function customprefix_prepend_text($title, $id) {
    if (is_feed() && in_category('custom-category', $id)) {
        $title = '<span class="custom-class">Custom text:</span> ' . $title;
    }
    return $title;
}