Here is the Product Docs on testing an Engagement Stream - Test an Engagement Stream - Marketo Docs - Product Docs
I'm not sure how to help you as far as targeting the right customers. I would build your smart list in the Lead Database and confirm that the individuals showing up are those that would expect to see. Possibly find someone that shouldn't be on the list and confirm that they are not.
When I create a new Nurture Campaign I add a couple test leads of my own to the program and run them through it. Triggering transition rules and flow steps to make sure everything is smooth before going live.
Always nice to have a way to remove someone from the flow of engagement if they move in a favorable direction in your lifecycle.