The Plugin manager has a feature to allow you to check what packages are installed when your creating scripts. For Example, if you create a script that relies on another script to be loaded, you can use this feature to detect them, ie, say you create a script that controls ship navigation, you could test to see if the Advanced Jumpdrive is installed, if it is, then it can make use of it, otherwise just do a normal jump.
To use the feature, the Plugin Manager includes a set of scripts, the one you need to use is the "plugin.plugman.checkpackage". You simply call the script, giving it the 2 arguments, the first is the Package name, the 2nd is the Package Author. The call script will return [TRUE] or [FALSE] if the package is installed.