Since you only want to use a vanilla server, you need command blocks. Why keep asking for commands here when you could do a simple google search and find all of them? You can find all commands here:
http://www.minecraftforum.net/forums/mi ... mand-blockIf there is a command you want, that isn't there, it probably does not exist. No need to ask here for commands that doesn't exist. There are also multiple youtube videos out there that explain how most commands to the command block works. Do some research instead of asking for everything
The idea of a mod checker is cool, but to make it work, you would have to make a system like rockstar games use on GTA V online, by scanning the directory and compare it to a fresh install of the game. This is something you can't do from a server, so a ingame automatic system will not be good enough to work complete. If you had a bukkit/spigot server, you could get a plugin that allows you to force users to "type" something in chat, including commands. Most hacked clients use (or used) commands to edit mod settings, and you could probalby get a plugin to read the error message. If there is none, it could be because the mod use that command, and show a custom error message from the client, and not the server. I don't know if it would work, but could be a idea to look into if you really want to make a mod checker.