How do you tell whether a share is undervalued or overvalued ? OR, How to Calculate if a Stock is Undervalued or Overvalued

How do you tell whether a share is undervalued or overvalued ?  OR How to Calculate if a Stock is Undervalued or Overvalued

This is Calculated by dividing the PE ratio by the EPS growth. 

e.g. If the PE ratio is 20 and the EPS growth is 30% then the PEG ratio will be 0.67. 

If the PEG ratio is low, less than 1, then the company is considered to be undervalued, while a PEG ratio greater than 1 suggests it's overvalued.