You know it's an email address when something's in the form email@example.com -- the '@' and the domain gives it away. Nowadays people know that @schuyler1d probably indicates a Twitter account by the '@' prefix, and #urlprefix is a subject tag.
But the most common Internet URI of all still has an unwieldy "http://" prefix or "www." too long. We used to be able to tell, because everything ended in ".com" (even when it didn't) but now URL shorteners and cool domains have hacked all the country codes to bring us delicio.us (which had to register delicious.com, because no one could tell that it was a URL), bit.ly (using Libya's registrar). And now ICANN is allowing a glut of TLDs (the ".com" part), even though no one uses them.... because everyone expects to type ".com"