It's fairly simple. You take a look at how they react to different things and how it affects them or their "goals". Everybody has "goals" in their life, as childish or as vague as they may be, and everybody subconsciously will act to further their own goals, or in a way that will benefit themselves, at the detriment of others. The furtherment of that "goal", is generally what the person will consider to be "good".
True "evil" does not concern itself with the "good" of others, only with theirs. Every event no matter how insignificant it is is viewed through the lens of whether or not it benefits their "goal" ; that, is how they define "good" and "bad". If it ends up helping someone else, it is usually pure coincidence, or with the goal of getting something out of it.
A "good" person will attempt to further goal that while not necessarily benefit them, or may even lead to worse conditions for themselves, because they have in mind the "good" of others.
My own father is a good example of this. He is pro-immigration and believes that everybody has a right to better living conditions, which might seem idiotic considering the dire consequences it has on the local population of our country (including himself). But as he himself states (and has shown), he is ready to pay this price, for he believes that these people need help, and if that is how he can help them, he will.
Is it misguided ? I believe so ? Foolish, even ? Perhaps. But it is a clear example what is the opposite of "evil".
That is also why i believe transactivism is "evil". Because it does not seek to help people, be they children or adults. It imposes itself as the only solution, disparaging even the mere possibility that someone may be misled into transitioning, or come to regret i later. Its goal is not to bring "good" to people, only to propagate itself and further the "good" of those that spread it. It is inherently a self-centered ideology.