Not an easy question. Fred BC and noah364 both make some good points.
I am going to work backwards, starting with the practical aspects of opinions vs facts, and then end with the philosophical.
Practically, an opinion is a view, belief, or judgment about a particular thing or claim, and it can be based on impressions, philosophy, previously held beliefs, observations, or any combination of these things. Opinions can be closely related to facts, or outlandishly antithetical to facts, as we view them.
A fact is what we call a claim that has been verified as accurate to a high degree of probability. The verification must occur on a minimum scale of measurement on which most would agree. There must be a ruler by which to measure the claim.
Very simply, the difference between facts and opinions is functional. Facts always work and remain constant, even if new evidence is introduced. Opinions do not always work, or it cannot be independently verified that they work consistently. New facts may expose an opinion as faulty, at which point logic would dictate a change in opinion is warranted.
The most universally accepted method that humans have devised to see what works and what remains constant is the scientific method, whether it is used formally by scientists or practically by individuals. This does not mean everyone will always agree about the results or their implications, but in the majority of cases in science, people have accepted the findings, used them, and found that they worked consistently and repeatedly. The widespread acceptance and use of the scientific method is why technology and medicine have made such progress in the last 150 years. The results are accepted because they work. I am able to post this online because the various claims of science about electricity, conductivity, computational physics, and many more phenomena work.
For example, very few people consider Newton’s law of gravitation a mere opinion, simply because it works. A great number of experiments and observations repeated over time by a great number of people indicate that there is a high degree of probability that a dropped object will fall to the ground. There are other aspects involved in movement and gravitation that apply outside of Newtonian gravity, but on a daily basis Newton’s law accurately describes how the world around us functions. It is a fact.
So, how do I determine what is true, what the facts are, as opposed to opinions?
Let’s use as an example how various people might view a single event, and inject some philosophical controversy just to make it interesting. An airplane with 200 people on board crashes violently upon landing, and there is only one survivor.
A scientist surveying the wreckage ascertains that the physics and mechanics of the situation would allow such an occurrence. Even though survival in such a crash is improbable, it is not impossible, and so it occurred within the normal laws of nature.
The person surviving sees it is a miracle of God that shows his mercy and power, and believes God suspended the laws of nature to make it happen.
A third observer sees it as evidence that the universe is random and unpredictable, without any power or God in control.
A fourth and fifth observer look at it as evidence of causality: the crash occurred because a series of events preceded it and made the crash inevitable. Observer four believes their is an intelligent power or force that set the events in motion, but observer five believes the accident is due to natural, physical occurrences that preceded the crash, without any guiding force or intelligence needed to set them in motion.
A plane with 200 people on board crashed upon landing, and one person survived. Those are the verifiable facts. All the other claims are opinions. However, not all opinions are equal as far as their relationship to the facts and the probability that they reflect what happened. The best I can do is to determine which opinion or opinions have the greatest probability of accurately reflecting what happened, based on the verifiable facts.
For me, if required to make a judgment on which of the above opinions were justified by the facts, I would have to look at the probability that they could be supported by the evidence in accordance with what we know about how the universe works. I would agree with the scientist that there is a high degree of probability that the laws of physics and mechanics allowed the person to survive. I would agree that there is a high degree of probability the accident occurred because a series of events occurred in the physical universe that made its occurrence inevitable at the point of the accident.
That is as far as my conscience allows me to go with any degree of certainty, based on the evidence and my experience of what works in the universe. Thomas H. Huxley described it as:
“ . . . the validity of a principle, which is as much ethical as intellectual. This principle may be stated in various ways, but they all amount to this: it is wrong for a man to say that he is certain of the objective truth of any proposition unless he can produce evidence which logically justifies that certainty.”
T. H. Huxley. Essays, Volume V
I would have no quarrel with any of the people who offered other opinions. I would have a quarrel with anyone who demanded that I acknowledge their opinion as a fact.
To address the question of whether opinions are subjective or objective, the question is somewhat misleading. Subjectivity and objectivity have to do with bias. Opinions, by definition, admit to bias, and though opinions may be informed by facts, they include a conclusion based upon the facts, not just the facts themselves. Opinions are always subjective, so the true issue is evaluating the probability that the opinion is accurate based upon all the available evidence.
Finally, from a philosophical standpoint, determining what is factual is a little more complicated. Why? Philosophy 101: the beginning of all discourse must start with an assumption about the nature of reality.
The easiest example is the classic “brain in vat.” Is the observable world that I experience real? How can I prove I am not just a brain in a vat, wired to a computer that is running a simulation that makes me believe I am functioning in the real world? Well, I cannot prove that I am not a brain in a vat being duped by some grand simulation because there is no way for me to step outside of what I am experiencing. I could say “I have hands so I can’t be a brain in a vat,” but my perception that I have hands could simply be a part of the simulation. Since I cannot prove I am not a brain in a vat, I must make an assumption. I chose to assume that the observable universe I am experiencing is real. It certainly seems that way to me, so it is more practical for me to act upon that assumption and get on with my life than continue to debate whether I am a brain in a vat.
Second, is my perception of the real world accurate? Distortions in perception can exist because of the limitations of our senses and our knowledge, and also because of misconceptions already held. The only test about the accuracy of our perception of reality is how our views work in the real world. We can also consider if our perceptions generally line up with the perceptions of others about what works. The shortcomings of both these methods are obvious, but the only other option is to ignore our experiences of what works in reality, and the evidence presented by others of what works, and create a totally abstract view of the universe in our minds. This is also the definition of psychosis - a detachment from reality.
My answer is, of course, an opinion.