How can i check the column which having all null values

We have a table named "TEST" with 4 columns (Col1, Col2, Col3, Col4) with following data -

1, Test, Exam, Online
NULL, NULL, NULL, NULL
2, NULL, Practice, NULL
NULL, NULL, NULL, NULL
NULL, NULL, NULL, NULL
NULL, NULL, NULL, NULL

How can i check the column which having all null values?