In a message's lifecycle, how to test all combinations of valid events, and see outcomes are covered or not.

Hello, 

Is there a tool or method which will show the untested flow combinations in a message processing architecture?

Let's think we wanted to replace a workflow with a reactive approach with messaging using service bus.

if different services are processing the messages in the service bus, is there a mechanism that all services and all combinations are covered with unit tests?

Such as there might we several branching in the message processing flow, and unit test coverage may show 100% as all symbols are tested, but there can be different combinations of execution paths. 

Thanks