I've made a game with a team in~ 6 month, we almost didn't play it. And this is an obvious sign the game isn't "fun". About your question, if part of your job

I've made a game with a team in~ 6 month, we almost didn't play it. And this is an obvious sign the game isn't "fun". About your question, if part of your job is game design or assurance quality, you have to play the game a lot. Like a player would.