You may not always be able to understand, but at least we have some ways to investigate, which is more than a lot of people do. We have the Buddha's teachings to guide us, we have the Four Noble Truth to guide us, we have the Noble Eightfold Path to guide us, we have the Four Great Efforts to guide us - which is part of the Noble Eightfold Path. We have the understanding that everything we think, say and do has a result. We have our past practice to know what is beneficial or not by observing in ourselves what is true happiness and what is suffering.
It is very difficult sometimes to know what the result would be, but if we can get our intention right, and if we can get our view right based on the Four Noble Truths, then we have a much better foundation towards seeing what is appropriate, what is true and what is beneficial. The Buddha encourages us as he did his son in the Rahulavada Sutta, to reflect, "Does this action, speech and thought I'm contemplating, will it lead to my own affliction, to the affliction of others or to the affliction of both? Is it a skilful mental, verbal, or physical action with happy consequences, happy results?" It is very difficult to get a good result from an unbeneficial intention. So when we try to use the Buddha's teachings as a guardian, we have much more ability to see beneficial results.