How can I determine if a linked record exists?

I need to populate a formula field. If the record exists on the primary table, I will use that value, if not, I will use a secondary table value.

In the past, I've been able to test the record key for nulls, and this seems to work. In this instance, I need to change the Report Option "Convert Database NULL Values to Default", so I'm looking for an alternate method to test for a record's existance.