Regarding if we should avoid disagreeing, Eliezer once wrote:

The central argument for Modesty proposes something like a Rawlsian veil of ignorance – how can you know which of you is the honest truthseeker, and which the stubborn self-deceiver? … the obvious reply:  "But I know perfectly well who the fool is.  It’s the other guy.  It doesn’t matter that he says the same thing – he’s still the fool."  This reply sounds bald and unconvincing when you consider it abstractly.  But if you actually face a creationist, then it certainly feels like the correct answer. …

Those who dream do not know they dream; but when you wake you know you are awake.  Dreaming, you may think you are awake.  You may even be convinced of it.  But right now, when you really are awake, there isn’t any doubt in your mind – nor should there be. … [you have] just an (ahem) incommunicable insight that you were awake.  … "That we can postulate a mind of sufficiently low (dreaming) or distorted (insane) consciousness as to genuinely not know whether it’s Russell or Napoleon doesn’t mean … [I’m] Napoleon. 

Are sleepers who think they are awake a good analogy for justified disagreement with fools?  Do sleepers and fools both have a broken mental state blocking them from assimilating key relevant info?  Interestingly, the merely sleepy do seem to know they are sleepy: 

Sixty-four adults participated in a study examining the accuracy of metacognitive judgments.  During 28 hr of sleep deprivation (SD) and continuous cognitive work. … Subjective and objective measures of sleepiness confirmed the expected patterns of increasing fatigue with SD. … Traditional indices of the confidence-accuracy relation (i.e., calibration, resolution, over- and underconfidence), as well as the accuracy of pre- and posttask estimates of performance, remained stable over the SD period. The findings suggest that people can accurately assess their own cognitive performance when deprived of 1 night of sleep.

Apparently it is also possible to know that you are dreaming: 

The first step to lucid dreaming is recognizing that one is dreaming. … Once … the recognition of dreaming occurs the dreamer must be cautious to let the dream delusions continue, but be conscious enough to recognize them … The rate that time passes while lucid dreaming has been shown to be about the same as while waking. …

Over time, several techniques have been developed to achieve a lucid dreaming state intentionally. The following are common factors that influence lucid dreaming, and techniques that people use to help achieve a lucid dream:

The wake-back-to-bed technique … involves going to sleep tired and waking up five to six hours later. Then, focusing all thoughts on lucid dreaming, staying awake for an hour and going back to sleep while … setting an intention, while falling asleep, to remember to recognize that one is dreaming, or to remember to look for dream signs when one is in a dream. …  A 60% success rate has been shown in research using this technique. …

Reality testing … involves performing an action with results that will be different if the tester is dreaming. By practicing these tests during waking life, one may eventually decide to perform such a test while dreaming, which may fail and let the dreamer realize that they are dreaming.  Common reality tests include:

• Looking at one’s digital watch (remembering the time), looking away, and looking back. As with text, the time will probably have changed randomly and radically at the second glance or contain strange letters and characters. …

• Flipping a light switch. Light levels rarely change as a result of the switch flipping in dreams.

• Looking into a mirror; in dreams, reflections from a mirror often appear to be blurred, distorted or incorrect.

• Looking at the ground beneath one’s feet or at one’s hands. If one does this within a dream the difference in appearance of the ground or one’s hands from the normal waking state is often enough to alert the conscious to the dream state.

Common dream signs include: 

• Falling.

• Flying.

• Inability to move one’s arms and/or legs.

• Dream taking place in a movie, book or video game.

• Clock faces, directional signs, books, etc., being unintelligible.

• Mechanical or electrical devices failing to operate in a normal manner.

• Gravity appearing to be stronger or weaker.

• Being attacked/chased by a monster.

• Being in a familiar area that doesn’t have the same layout as it does in the real world. For example, being in Grand Central Station, except it has the interior layout of the Gare du Nord.

• Having sex (see wet dream).

• Being lost in a building (even in a building that you are familiar with in real life).

• Familiar sights, such as the faces of others, or of one’s own face in a mirror, appearing distorted.

• Interacting with friends, relatives or family pets that are deceased in reality.

• Being naked in public, and being the only one who seems to notice.

• Being impervious to injury (i.e. feeling no pain), especially of the lethal variety (for example, being unharmed by an explosion or a gunshot)

All this work to collect dream signs – can it inspire us to collect "fool signs" that tell us when we are wrong to disagree?