Is there a Captivate system variable that tells me if the project is playing?

I know of no way to detect if the movie is currently playing or not (without any events occuring), but you can listen for the events to pause or resume it and then e.g. update a user variable whenever that happens (or call any other function for that matter).

Create a user variable v_isPlaying with a default value of 1 and run the following javascript on slide enter:

cpAPIEventEmitter.addEventListener("CPAPI_MOVIEPAUSE", function(){window.cpAPIInterface.setVariableValue("v_isPlaying","0");});

cpAPIEventEmitter.addEventListener("CPAPI_MOVIERESUME", function(){window.cpAPIInterface.setVariableValue("v_isPlaying","1");});