Autocorrelation occurs when the residuals are not independent of each other. That is,
when the value of e[i+1] is not independent from e[i].
While a residual plot, or lag-1 plot allows you to visually check for autocorrelation, you can
formally test the hypothesis using the Durbin-Watson test. The Durbin-Watson statistic is used to
detect the presence of autocorrelation at lag 1 (or higher) in the residuals from a regression.
The value of the test statistic lies between 0 and 4, small values indicate successive residuals
are positively correlated. If the Durbin-Watson statistic is much less than 2, there is evidence
of positive autocorrelation, if much greater than 2 evidence of negative autocorrelation.