It is impossible to test a deployed application in its running state by using existing testing facilities which are aimed at building time testing. Instead, we should implement means to the codebase in order to access the runtime state of the program. This includes using logging, hot-key sequence (opens a pop-up diagnostic control window) and web server.
For logging, we need to trace the state of the program in the form of messages. It should be in a regular and consistent format so that it is easy to read and parse by other programs. Information such as timestamp is great to have to know when did the state changes.
We can deploy a web server to view internal entries and log status of the program with GUI instead of looking into files. It makes runtime diagnostics easier in operational terms. If possible, having a debug control panel can come in handy.