John, if your document is stored with the decimal field equal 0 (or 0.00) then you may seach for it by placing 0 in start range and another 0 in the end range of your decimal field in the search dialog.
BUT, if your field is stored without any value (NULL in database), you cannot search for it by using a search dialog. But if you have Tasklist module, you can create a list and specify filter criteria as Decimal Field = "is empty" - this will show you only entries entered as NULL.. If you want your result also to include 0 amounts, use this filter criteria in your list definition: DecimalField = "Is (where claus)"=EMPTY() OR 0