How can you tell which items are unscored and which ones are scored? You can't. That would defeat the purpose of the pretest items. Most people would skip pretest questions if they were marked as such. All you're left with is a guess. But there's something helpful about that. If you encounter an exam question that leaves you baffled. "What are they talking about?" or "Am I really supposed to know that?" then you can take your best guess without having your confidence sunk. Say this: "That one was probably unscored. Next!" That's the gift of the pretest items. You can be totally clueless about 20 test questions without having to worry that you're clueless about what matters on the exam. (Will there really be as many as 20 baffling questions on the exam? Probably not.)
Hopefully that clears it up. Find more info on the ASWB's website. But, really, you don't need more info. You didn't even need to know this. Learn your social work, take some practice tests, and go get licensed! Good luck!