A simple analogy to make a simple point.
How do you know if the currency you have is real or not?
According to this article, there are a number of ways to determine a bill is real our counterfeit:
3D security ribbon to the left of Benjamin Franklin’s portrait visible only on the front of the bill. Holders moving the bill around should see blue Liberty Bells that change to the number 100. Depending on whether the bill is held and tilted vertically or horizontally, the images move back and forth or up and down.Now, applied to a person: how do you know if a person is good or not? We read about this in Discourses 2.1. We can know if a person is good if they "never fail to attain what [they] desire, ... [they] never fall into what [they] want to avoid. Bring death before [them] and you'll know. Bring hardships, bring imprisonment, bring ignominy, bring condemnation" and they will view these as indifferents.