title: unique identifier in data.table
body: <p>I have a <code>data.table</code> with 11 variables and 200,000+ rows. I am trying to find the <code>unique identifier</code> (in other words, <code>key</code>) in this <code>data.table</code>. </p>
  
  <p>I am looking for something like <a href="http://www.stata.com/help.cgi?isid" rel="nofollow">isid</a> in Stata, which checks whether the specified variables uniquely identify the observations. Can someone please help?</p>