Arquillian and method execution verification

I'm running my .ear in container running arquillian. I'm testing some communication with external system, so I have created simulator for that system.

I want to be able to check the format of passed messages. How should I do it?

Should I put asserts into the simulator or is there a way in arquillian how can I spy on some class? Or replace some specific class with mock class which can play the role as simulator.

There are no EJB nor CDI injection. It is just some reqular class, which I cannot set manually. Any ideas.