Step 1: When you create a new connection, for example as shown here in Connect To hadoop article, you will get to Preview Screen.

 

Step 2: On preview screen, on top of every column name you will see a horizontal statistical chart which will show you the data quality of that column 



 



Step 3 :  It can be interpreted as below-

Green - number of values Parsed Successfully

Red- number of values don't parsed

Yellow - number of missing values




 

Step 4: You can select the one which are null or invalid and it will highlight those column values and its corresponding values. System will also suggest you operations which you can possibly perform to improve data quality in left suggestion panel.