Easy, call a clientCmd, make the clientCmd check it.
Thats a start.
I would do something like this:
1) in ::onClientEnterGame, you set a variable to "false" or "no" or something to indicate "player does not have my mod installed"
2) also in ::onClientEnterGame, you do commandToClient(%client, 'CheckMyMod' ....
3) in client.cs (which they have downloaded).
put the code for: clientCmdCheckMyMod - all it does is something like: commandToServer('MyModIsLoaded')
4) in server.cs put code for: serverCommandmyModIsLoaded(%client). in that function you set your variable from step 1 to "true" or "yes" or something to indicate "player DOES have my mod installed"
5) anytime the player tries to do something you check your client variable, and if its set to no mod loaded, then you send the client an error.