Realizing my assumption about Italians is not based on anything substantial uncovers a falsehood, or at the very least a hastily-drawn, unsubstantiated conclusion. — Tzeentch

Well yes, but that's trivially true and I can't see anyone disagreeing, who's going to argue that being knowingly prejudiced is a good thing? The reason why such arguments never work is that they presume one person's view (usually the protagonist) is sufficient for defining prejudice. If one person thinks all Italians are lazy, it is rarely because they have drawn what they themselves consider a hasty prejudice, at least not when we extend this metaphor to actual philosophical positions. So an argument which should be about the utility of each position instead dissolves into a rhetorical game of being the first to apply the label of 'bias/prejudice' and get it to stick.

The op is just another example of such rhetoric. I don't deny that some philosophers are prejudiced, and I don't deny that many are only interested in the maintaining the status quo, but I would argue that we do not have any reliable means of determining which.

When is Philosphy just Bolstering the Status Quo?

When it's primary concerned with what should be, instead of what is. Or in other words, good philosophy starts with accurate description, not with proscription.

but I would argue that we do not have any reliable means of determining which. — Isaac

Yes we do, if they make bad arguments or weird assumptions just to come to certain conclusions. Then we know they just want to prove their prejudices. Kant for instance, was evidently very troubled by Hume, and pulled out all the stops just to retain the idea of God.

There are a lot of people who have such prejudices, most often unknowingly. Things one has been taught from childhood and were never properly scrutinized. My example was an obvious one, but when we start uncovering similar (less obvious) predispositions, prejudices and false assumptions about the self, reality, etc., and one will start to see how big the web of illusion is that we've spun for ourselves. Indeed, these may not seem like falsehoods until we properly examine them.

It is exactly this process of dismantling one's false sense of reality that led Descartes to his famous proposition "I think, therefore I am", which was the only thing he felt he could be certain of. Similarly when the Oracle of Delphi proclaimed there was no Greek wiser than Socrates, she was right. For Socrates knew just as much as any other Greek; Nothing. Paradoxically, Plato's account famously stated Socrates saying "I know that I know nothing", and later "I seem, then, in just this little thing to be wiser than this man at any rate, that what I do not know I do not think I know either."

Long story short; We have a lot of illusions about what we think we know, and one cannot build philosophy upon illusions.

We're going round in circles. You keep talking about "false" assumptions, I'm just asking how you know they are false prior to determining what 'falseness' actually is.

In science, if I predict a phenomenon will act in a certain way and it consistently does not, I discard my theory because consistently failing to act as I predict is my definition of 'false'. If I continued to believe in the theory as a scientific truth despite this failure, I could be said to have a prejudice.

What is the equivalent test in philosophy, and what is the proper field of investigation responsible for determining what this test should be?

In that regard there is not much difference between science and philosophy. However, in science others can verify one's findings, which isn't always the case for philosophy.

How do we know when philosophy is just justifying the status quo of whatever is considered important for society to function. — schopenhauer1

Much is socially constructed...to wit, if you must believe in the existence of something before it exists (money, say)...it is a dubious claim on the truth, a false etymological position, which leads to blank, mimetic imitation of what everyone believes without any good reason (status quo). Most of what our species collectively takes for reality, can't possibly be reality because it is based on invisible, untenable systems of rule (money as a medium of social construction) (when an individual has invisible beliefs, or idiosyncratic motivations, that no one else has, he is considered deviant or perverse; but when a group of people agree to live by an agreed upon system of unassailable beliefs, it becomes the neurotic norm). Mechanistic science and peer-review, insofar as these beliefs are utterly deterministic to the perspective of the scientists, is redolent of how the collective belief in money makes it real. Where does social agreement (status quo) fall on a spectrum of "wisdom of the crowd"......to, say Freud's or Le Bon's views on the "popular mind/crowd psychology" (social constructed reality tending to lead to mimicry and diminishing of mental powers instead of being responsible for having a fecund, original mind)?

Lately, it has become apparent to me more potent philosophy is mainly pessimistic and skeptical inasmuch as it is all too easy to fall into an intellectual cul de sac of unexamined, collective agreement (derived from mindless groupthink without any close scrutiny to the content or context of such mores). Thus, fitting in socially precludes any real philosophizing. Sociocentrism, with its irresistible gravity to distract oneself from alienation, is all too often the first step away from philosophy concerned with subjective and objective veracity as a whole ( a phenomenon rampantly increased by impersonal communication, viz, telecom). An example, take scientism: starting with the enlightenment, the majority of people, perhaps straying from truth, don't accept reality unfiltered through logic and metrology (this being the status quo up to current). If we all have the same ruler, it's impossible to miss the truth, right? Or does it become impossible for us all not to miss exactly the same aspects of truth? People are ignorant of the same facets of truth as a result. This facet swells over time.

What's logical is informed by what's illogical; what can be measured is informed by what can't be; what's impossible is the context of what's possible; the thinkable is surrounded by the unthinkable, and so on. Believing apprehensibility of the truth is limited to one side or the other of these mutualisms is to project a standardized system (of agreement) onto the truth and thereby overlook it.

pick any philosopher that says hard work is good or meaningful in itself, — schopenhauer1

But what's an example of that, for example? Maybe you're reading different philosophy than I am, but I don't see anything about "hard work" pro or con very often. The only thing I can think of offhand in that regard is Russell's "In Praise of Idleness", and even with that, we're talking about something almost 85 years old already.

Honestly it is what I've seen on these forums. This type of response inspired me:

https://thephilosophyforum.com/discussion/comment/241534

https://thephilosophyforum.com/discussion/comment/241539

Basically it is not questioning what is the given. Even the idea that "society must flourish and continue" is not a given. You must question every perspective. Perhaps society is always a harm to the individual. The consequence of course is to not use an individual for society's benefit, even if there is a symbiotic relationship. That leads to other conclusions, etc. But that would be radically challenging what is thought to be dear, that is to say, the status quo.