The normal probability plot indicates whether the residuals follow a normal distribution, in which case the points will follow a straight line. Expect some moderate scatter even with normal data. Look only for definite patterns like an “S-shaped” curve, which indicates that a transformation of the response may provide a better analysis.
Formal statistical tests are not used for this diagnostic; visual inspection of the graph is sufficient.
Externally Studentized residuals are the best metric to use for this plot. Internally Studentized, and raw residuals are available but not recommended.