DISCLAIMER: This project is experimental
ModObserver plugin can be used to monitor, what mods are the players using. The ModObserver mod is required to be installed by the players.
Here are the configuration options for the plugin:
In-game
Whitelist: pretty self-explanatory
Blacklist: inverted whitelist
Required mods list: Mods, that the players must have installed.
Ignored players list: Players, that are excluded from the mod checking process.
Manual player checking: If you are not sure, if a player is cheating, you can use this feature to check them. You can also choose, if you want to kick them, if they are using prohibited mods.
Verification timer: Makes the players send their mods not just upon joining, but even when playing. This feature is turned off by default.
You can find those options under the /modobserver command.
In the config file
Options are the same as in-game, and some more:
Kick messages: You can change, what is displayed to the players, after they have been kicked out, by the plugin.
Turn off configuration command: If you don't want ops to have access to the plugin, you can turn off the /modobserver command.
Just to clarify, this plugin does NOT guarantee, players won't use disallowed mods. This plugin is just another layer of protection you can add to your server.