The only way I can think of to test that is to have someone make a decision, then go back in time and see if they can make a different decision
In my opinion, though such speculation is fun to think about, it's not really important. More importantly, I don't consider the split-brain experiments to be compelling evidence for this, just that our left-hemisphere tends to rationalize with whatever it can get, it doesn't really say anything about free will--again, it just relates to how it works.
I think it's important precisely because of the notion we have that it is unimportant, if that makes sense. We feel that we have free will and we shrug these kinds of questions off. We don't really like to think about it. But when you do think about it, our conception of science suggests that consciousness is a quantity, while in everyday terms we treat it as a quality. The quality might just be after-effects of the quantity, is what I am saying. I think it is applicable in the sense that it would make us more aware of how our beliefs are grounded, and what is "correct" vs. what we believe in for x and y reasons.
An interesting talk on the subject:
http://www.ted.com/index.php/talks/dan_dennett_on_dangerous_memes.html