title: pandas retrieve the frequency of a time series
body: <p>Is there a way to retrieve the frequency of a time series in pandas?</p>
  
  <pre><code>rng = date_range('1/1/2011', periods=72, freq='H')
  ts =pd.Series(np.random.randn(len(rng)), index=rng)
  </code></pre>
  
  <p>ts.frequency or ts.period are not methods available.</p>
  
  <p>Thanks</p>
  
  <p><strong>Edit:</strong>
  Can we infer the frequency from time series that do not specify frequency?</p>
  
  <pre><code>import pandas.io.data as web
  aapl = web.get_data_yahoo("AAPL")
  
  &lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
  [2010-01-04 00:00:00, ..., 2013-12-19 00:00:00]
  Length: 999, Freq: None, Timezone: None
  </code></pre>
  
  <p>Can we somehow can the aapl's frequency? As we know, it's business days.</p>