how to check null values in dataset?

Question

how to check null values from dataset?

  if ((ds != null) && (ds.Tables.Count != 0) && (ds.Tables[0].Rows.Count != 0) && (ds.Tables[0].Rows!=null))

 i got a dataset from excel file

 dataset 
 1. hardik
 2.Mahesh
3. null
 4.null
5.rajesh

i want to find that 3rd and 4th rows from dataset