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.