You could enumerate the global properties, functions, and types and look-up the namespace of each.

Finding out if a function accesses a variable, function, or type from a different namespace is not as easy though. With some work it ought to be possible to enumerate the instructions and verify the entities accesses using the same interface used by JIT compilers.

What do you need this for? If it is something that could be useful for others too I'll evaluate implementing some interface to make it easier to enumerate used namespaces.