You really cannot tell if one person is kinder than another?
I can make my own personal assessment. That
is not objective.
Like, my wife is a veterinarian. Sometimes, she has to help animals pass on. She uses the most humane drugs available, and works to minimize the distress of both her patients and her clients. I would, personally, say that in ending animal suffering, she's engaged in an act of kindness. In sum total, my wife is likely the kindest person I know. And most veterinarians are in the same category.
But, there are folks who feel that ending
any life before it passes of its own accord is an act of cruelty - that life is so precious that no matter the pain or distress involved, it is unacceptable to end it. I
think they are wrong, but there is no source separate from human feelings or belief systems I can turn to to demonstrate that as a fact.
So, no, I cannot claim there is any objective assessment of kindness. We, as a culture, cannot agree on what constitutes a kind act. And even if we did, some other culture might disagree. For it to be objective, it must be true
beyond culture or personal feelings on the matter.
You really cannot tell if one person is more intelligent than another? You can take that as "better at a given intellectual task" if you want to avoid debating the existence of general intelligence.
Except, by making that change to one specific given task, you have changed the question so that it isn't about intelligence - many tasks that we might attribute to intelligence, when taken alone, can be done more quickly by rote memorization of patterns, rather than doing analysis. Speed solving a Rubik's Cube is one example. It is about having perfected some specific way to do that specific task. So, it no longer clearly indicates what we were talking about, and is therefore not relevant.
You really cannot tell which events in history had greater influence on downstream events than others?
Objectively? Again, no. If you ask several professors of music history who was the single most influential musician of all time, you will not get unanimous agreement, which should be the case if this were an objective fact. Indeed, an alien from Alpha Centauri would also have to agree, if it were objective.
With respect your inability to tell such things does not mean there are not objective differences that can be recognized and assessed and neither does the absence of universally agreed upon units of measure.
For a thing to be objective, it must be free of influence of any personal feelings opinions, or biases. As noted, "kindness" is a
human cultural and emotional construct. It is
founded in human feelings, and therefore cannot be objectively defined.
All acts of kindness must be assessed with respect to the mental states of the actor (which you may not have) and the recipient (again, you may not have that), the actual results of the act (which you may not know) and your own moral/ethical codes. There is no way that this assessment, generally made in ignorance, and passed through several subjective lenses, can be an objective thing.