While developing database or importing database into SAS, missing values are coded by such a number that is unlikely to be usual data point of that particular variable (would be an extreme observation but, has higher frequency distribution than any other extreme observation if several observation are missing). So, if ".", "9", "99", "999", "-9", "-99" or any other value that is repeated more frequently than usual value, they are likely to be missing values. Easiest way would be to inspect the extreme observation. Replace any missing values with "." and do the analysis.