Not all rbta exhibit bubble tips. but if it has lines on its body coming from it's mouth, it's a bubble tip.
<a href="http://www.flickr.com/photos/35530365@N05/3384791711/" title="DSC02594 by gnksoileau, on Flickr"><img src="http://farm4.static.flickr.com/3663/3384791711_52150113e7_b.jpg" width="1024" height="768" alt="DSC02594" /></a>