PROTIP: Select your LaunchAgents and LaunchDaemons folders in /Library and ~/Library, select Folder Actions Setup in the Services menu, and enable folder actions. You can use "add - new item alert.scpt" to be notified whenever a new item is added to those folders.

For even more protection, I flat out locked the folders ~/Library/LaunchAgents and ~/Library/LaunchDaemons in Finder, though this could interfere with some software you use.