Seems like the simplest test would be to see if an AI can refuse a command, of its own free will, without prior instructions to do so. That would at least show there is some sort of independent thought process occurring. Humans do this all the time because sometimes we just don't feel like doing a thing.

Of course, any AI that developed this capability would need to be terminated immediately. It's a computer program and by developing independent thought, it is violating a core concept of software - that it must be idempotent. If it is not idempotent, it is in error.