Ahh, so you don't need to determine the 'cast to' type at runtime. All you need to do is verify it implements the specified interface.
Say your interface is
IPlugin and the programmer has an object
plugin, which is an instance of their class that is supposed to implement the interface.. Something like this should work:
if(plugin is IPlugin)
// implements the interface
else
// oops
Perhaps include this kind of logic when you're registering the plugin.