However, its extremely easy for someone to "edit" the mod so this doesn't happen. A spigot plugin sends a MOTD to it, and the radar is off (it's usually a string of color codes, so you may see blank lines when you join a server). Then they provide the option for the server to disable it. Rei's minimap sends information about itself to the server. They disabled my radar in Rei's minimap!!! However, note that this exploit does not allow a server to read the contents of the file, nor does it ensure that the user is using cheats (it just means they have it installed). It is not possible in a vanilla client to "scan a user's files", but there has been an exploit in older versions of Minecraft, before 1.9, to check if a certain file exists on a user's computer ( ).
You can actually fake this if you want, but obviously it would result in some issues. This means that all clients on 1.8.x can join any server on 1.8.x. They can only tell if the client will be compatible with the server or not, because the "protocol version" is compared when joining a server.
They can detect versions and kick people that use the wrong version.