How can I tell if a field has spaces or nulls?

[smoothyc]

I have two fields in a table "Item" defined as varchar2(50) and "udc_drp" defined as varchar2(50)

"Item" field contains the following data:
"123456789 .......spaces up to 50"

"udc_drp" field contains the following data:
"Company ........spaces up to 50"

When I do a select on the item field my rows are returned. When I do a select on the udc_drp field, no rows are returned. However if I pad the where clause with spaces for the udc_drp field, to equal 50, my rows are returned.

Why the different? What could be causing the problem?