Minecraft wiki entry for Mods, associated text: The code that checks for mods is fairly simple, and it's not always correct; it may say 'probably not' even with mods installed. However, it's very difficult to get the 'very likely' message if you haven't modified your Minecraft .jar file somehow, so that's essentially a 'yes'. There's also a 'definitely' message, seen when a Bukkit server crashes and under other similar circumstances, like when the Minecraft Forge API is installed.