It's just so hard to tell...because someone who is really good at one subject may get a bunch "hard" questions in that subject, but they handle it well. Then they get "easy" questions dealing with subjects they're shakey on. That's the making of a fantastic score right there.
But on the other side, what if you get hard questions where you're shakey? And the easy questions are the subjects you know best? Very frustrating.
The answer: don't be shakey! Be solid!