Minecraft wiki entry for Mods, associated text: 'Probably not' appears when the client/server brand appears to the in-game check to be unaltered (often termed 'vanilla') and the META-INF folder is still there. 'Very likely' appears when the META-INF folder is not present but the client/server brand seems to be vanilla. 'Definitely' plus the client name appears when the client is not vanilla: