It is currently impossible to distinguish between a NULL value and a string that happens to be spelled, "null" because LinqPAD displays "null" (lowercase) for both of them. SSMS displays NULL (uppercase) for NULL values, and to differentiate between NULL values and a string spelled, "NULL", SSMS highlights actual NULL values with a special background color (a soft yellow). LinqPAD should do the same thing, or something similar, to make it more obvious if the value is NULL or a string that just happens to be spelled "NULL".