[Altus]

You can make a custom launcher, but I don't think that you can force the players to use it, preventing joins with the help of the default one. Of course, you can also write a tool which would scan the contents of the GTA SA directory of each player and send the data to your server, and to banish those who dared to install anything illegal, but... Is it worth the hassle? I doubt that the players would be happy to be forced to install anything before joining your server. Personally, I wouldn't bother and would just go to the next server in the list.

So, as BiosMarcel has already mentioned, better think of a decent anticheat.