In my experience, a lot is based on what they've been exposed to. I can tell you that for homeschoolers taking annual standardized achievement tests like the CAT (California) or the ITBS (Iowa), social studies is often lower than anything else because the kids studied other things--the Norse or the Greeks, say--rather than what's on the test.
For example, there was a question my son had once about a crossing guard. Well, he's a homeschooler in a suburban area where school kids ride buses. He's never seen a crossing guard, nor had the subject of crossing guards come up in conversation. He missed that one and several others along the same lines.
I'd imagine the same sort of thing would happen with an unprepped kid being tested on the school's curriculum unless she just happened to have stumbled across those particular bits of info in her life.
Science seems easier for them to pick up through experience and independent reading. (I'll admit that DS8 is a very science-minded kid, though, so that may be my own bias. But then again, I don't know very many social-studies-minded kids. History, yes; crossing guards, no!)
I'd recommend finding out what curriculum they use for social studies and science. If it's possible to get some assessment tool for that specific curriculum, that would be ideal. I doubt it's possible, but it would be where I'd start. I don't think assessment tools will be very helpful if they're designed for other curricula. The info just tends to be too specific.
