So how do you know if you are too open-minded? How do you know if your brains are falling out? Here's a simple test for open-mindedness that everyone should take when thinking about an issue. I saw this idea on a comment board somewhere at skeptic.com and had to borrow it. You'll find that it's spectacularly useful in cutting through layers of confusion, for debating issues, weighing reasonable arguments, etc.
The test is this: Pick a side of an argument (if you haven't already). Then ask yourself, what would it take to convince me that my position is incorrect, that I am wrong? What evidence or argument would it take to sway you to the alternative?
If your answer is "I dunno" or "Nothing would convince me" or some impossible evidential standard, then maybe you are not really being open-minded, reasonable, rational, critical, or appropriately skeptical. You certainly aren't being scientific, which requires empirical testing of all claims for refutation or verification. If you aren't dealing with a claim that is testable or falsifiable, then it is not a scientific hypothesis or theory, it's just a plain old regular everyday unsupported belief with nothing to back it up except more belief. If this is the case for your side of a debate, be open-minded enough to consider the possibility that perhaps your brains are falling out.
Let's take Bigfoot as an example. I don't happen to believe in Bigfoot, I think the good solid compelling physical evidence just isn't there when it obviously should be. But I am more than happy to admit that I would gladly change my mind once solid irrefutable physical evidence appeared, like a body or major body parts (dead or alive or fossilized would suffice). What would it take a Bigfoot believer to disbelieve, to change his/her mind? There is almost never an answer given to this question, as it's never even considered or entertained by most. I sometimes find myself asking what's the point of arguing, or really of going over evidence and reasoned debate, since so many people are never going to change their minds anyway. They aren't swayed by evidence because their mind is made up, or they give proportionally-greater weight to evidence that is proportionally-crappier than real forms of actual evidence. And that's the problem. More often than not, in pseudoscience and in life, believers just want to believe. And that's not being open-minded, that's the close-minded version of your brains falling out.