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[Guest]

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Edited January 23 by D0XY

[Cadiboo]

I know that the mods a player is using are sent to the server on connect, you could find where that code is & then see if/where that information is kept/used

[diesieben07]

I am assuming there will be some kind of communication between server and client through plugin channels.

I am not sure about Bukkit (assuming that's what you mean by Plugin), but I assume it has a method to check if a plugin channel is registered for a player, just like FML (FMLNetworkEvent.CustomPacketRegistrationEvent). So to check if your mod exists on the client, simply check if it's plugin channel is registered.