As the complexity and scope of games increase, game testing, also called playtesting, becomes an essential activity to ensure the quality of video games. Yet, the manual, ad-hoc nature of game testing leaves space for automation. In this paper, we research, design, and implement an approach to supplement game testing to balance video games with autonomous agents. We evaluate our approach with two platform games. We bring a systematic way to assess if a game is balanced by (1) comparing the difficulty levels between game versions and issues with the game design, and (2) the game demands for skill or luck.