Both are different cases.
If you want to check all the values whether it's null or not
use
select * from TableName
and if you want to check all the values which are not null.
use
select * from TableName where ColumnName is not null
But i don't think that you want to ask this, as even a beginner in sql knows the how to use is null and is not null in where clause.
so please clarify what you want to achieve and provide sample table, data and your goal.