I always expect Drupal comments to bear at least "new" tag, or a different coloring for the specific comment, if it hasn't been read by the user already. This isn't working on this forum and I'm usually compelled to goto #new fragment and ride the complete discussion :P
6 Answers
Check for $new usage in comment.tpl.php. If it isn't there, copy the tag line from garland and adjust it (would take 30 seconds). Even otherwise, you may want to pastebin comment.tpl.php so that I can suggest something decent.
I tried looking in the comment.tpl.php but it looks totally different from garland theme. Here is the comment template. The theme is Marinelli. May be you can submit a patch to the theme :)