Difficult question to answer.
It goes without saying that a lot of the intelligence tech that could nail it down tight is going to be classified.
Things to keep in mind:
--North Korea is famous for their "Potemkin" efforts in many areas.
--North Korea is making a big deal out of "no radiation released."
Points in favor of the test being fake.
Now, if you ask me what the possible motivation could be for North Korea to fake a test and bring the wrath of the world down on their heads, you've got me. That's a very big point in favor of the test being real.
Waiting to see on this one.