If you were asking from the perspective of working this out with a running program then you need to look to the java.lang.* package. If you get a Class object, you can use the isAssignableFrom method to check if it is an interface of another Class.
There isn't a simple built in way of searching for these, tools like Eclipse build an index of this information.
If you don't have a specific list of Class objects to test you can look to the ClassLoader object, use the getPackages() method and build your own package hierarchy iterator.
Just a warning though that these methods and classes can be quite slow.
