As someone with a background in Chinese history and China studies, this is a dilemma I am faced with often. Firstly, it does take a bit of diligence on your part to determine the plausibility of any one specific claim. Checking sources, citations, authors, etc. is always a good way to start. Academic university presses force all of their potential publications to go through peer review, so you can be relatively confident that the material you read through these sorts of publishers is at least relatively well-vetted. On the flipside, if you are faced with a source of unknown provenance, it might be a good idea to check for the source's background. Authors may bring biases into their work which may or may not impact the works they create. One of my recent reads, The Dragon's Gift by Prof. Brautigam from Oxford Press, is IMO an example where the author's relatively positive disposition towards China does not compromise the integrity of her research and writings. On the flipside, if you were to take a look at a work like The Black Book of Communism, it becomes quite clear quite quickly that the biases or predisposed notions of the contributors taints the quality of scholarship considerably. Keep your head on a swivel and bring a questioning attitude to anything you are faced with. I know relatively more when it comes to China than I think the average person and when I read through books on China, I can leverage my baseline of knowledge to assess what I'm reading. I know relatively little about, say, western military history so when I read something about World War One, I tend to be more careful both when I choose what material to read and when I actually read the material itself.

Beyond that, in my opinion, I think it's simply not good enough to ask the question in plain terms. Well-constructed arguments are rarely simply just 'good' or 'evil', 'right' or 'wrong'. Often, reality is much more complicated than simply painting important people or events in a two dimensional manner. Within my field, there are many personalities who have experienced rapidly changing character assessments and whose accomplishments, both good and bad, require more thought than simply being labeled as purely noble/good or evil/ineffective. People like Mao Zedong, Ming Taizu, Tang Taizong, Prince Dorgon, etc. have been enormously influential and are very complicated. It would be doing not just the historical record an injustice but also your own learning a disservice to simply say something like "Hmmm Dorgon instituted the queue order and enforced it with capital punishment, I judge him to be completely evil" and move on. In order to get a better understanding when assessing statements like you describe for anyone really, it is important to acquaint yourself with as much well-sourced information as you can and then maybe come to your own conclusion. Ultimately, I think it comes down to familiarizing yourself with the topic and making your own, well-informed assessment. Looking at simple and boring arguments can limit your horizons and is generally not very helpful.

TL;DR: Do your due diligence when analyzing a source you're looking at, think critically when reading it. Read more to broaden your knowledge foundations.

I hope this can somewhat answer your question

These aren't really facts though. They are interpretations of historical figures' lives reduced to a few extremely loaded words. There is no reason why Che Guevara couldn't have been both racist, murderer, and freedom fighter during various periods of his life. There might be reliable sources documenting racism, but there might also be evidence for him fighting in Africa to help free Africans from colonialism, which could suggest a change in him or just that he contained both sides. It will always depend on a historian's interpretation, and here it's the historian's political views that might influence how generous his interpretation of Che Guevara will be. Still, a good historian will not go much beyond the sources and be clear about when they are speculating. Over time the generally accepted interpretations will often change a bit as society changes, as new events make history be seen in a new light, and new sources might even appear to give an edge to one interpretation over another, but it will never be a simple either or.