Senior Member. It's another way of saying something like "If you had asked me earlier, I would have thought that you knew". The "should have thought" tells us that he didn't actually think this in the past, but is imagining what he would have thought.

Would have thought or would have think?

As I've learned that "I would think" is used for giving openion ,politeness , showing uncertainty,softening etc "I would have thought" should also share the same characteristics as It's only past version of "I would think". Example: A: She died of a broken heart.

Should have thought of that meaning?

I should have thought ... British English used as a polite or joking way of showing that you disagree with what someone has said or think it is silly 'Why isn't it working? ' 'I should have thought it was obvious.