Java Game Developer Question:

The only real way we have to test such things is with, well, testing. Professional game developers will tell you all the time that the single most effective way to know if certain gameplay is working and fun is to give it to players and observe.

Indeed, one of the reasons that achievements are everywhere these days in games is because they give invaluable information to the developer. If you want to know where players stopped playing your game, make an achievement for completing each level. If you want to know how many players aren't playing a certain race in Civilization, look at how many people got the achievement for playing that race. And so on.