Question
How we can be sure that Krishna (God) is absolutely good, without any trace of evil?
Answer

I know that Krishna is ultimately good because throughout my long life, I constantly see that Krishna rewards virtue and allows us to suffer the natural, fair consequences of non-virtue.
The fact that the world is fully of beauty for those who have eyes to see it, the fact that goodness, kindness, generosity, and love bring us real happiness, and selfishness always leaves us empty and frustrated—all that teaches us about the nature of our Creator.