In certain circumstances, something can be good, but in others, the same thing can be bad. There is no absolute.
We apply our judgment based on the particular circumstances. Normally we consider that any action that gives us happiness is good, and what creates sadness or pain is bad. That is to say you know what is good or bad is based on experience.
Our spirit has the last word.
[The Little Book of Wisdom of the Dalai Lama]