As bobdoering has stated in his post, the measurement system should be first assessed whether it is acceptable. That should be the first premise in the data analysis.
Provided that the measurement system is deemed to be adequate, a test for normality would then ensue. As others have mentioned, the p-value from the p-plot can be used to assessed whether the data follows a normal distribution. A p-value < 0.05 indicates a poor fit to the normal curve.
Normality testing is very important, the Cpk being reflected by Minitab assumes a normal distribution. The index, however, would then result in a misleading analysis of how your process is performing.