Over the past three years, I've created and waded into a few ColdFusion and Flex applications, good and bad. 

The good all have a common theme:  unit tests (integration tests, really) that test the actual CFC endpoints the Flex application uses.

The bad all have a commont theme:  no testing at all, or unit tests that test the domain model (and possibly some services).

There's enough "weirdness" occuring at the level of transport between ColdFusion and Flex that I've found it invaluable to write tests that simulate what the RIA client is sending / expecting as exactly as possible.  It's impossible to do perfectly at first - only experience teaches every oddball conversion between Flex/AS3/AMF and ColdFusion, esp. w/ CF9 ORM involving nulls - but it's important and an invaluable timesaver.

If you're not testing the actual CFC endpoints to which Flex speaks (the "source" part of your RemoteObject tag), grab some MXUnit and get started!