Jesus often talked about the vine, tree, and fruit. For example, you won't find an apple growing from an orange tree, and you won't find figs on a grapevine. The fruit of the tree tells us what type of tree it is...it reflects or represents the tree.
The Bible tells us in Genesis 1 that everything God created was "good". The Bible also tells us that it wasn't until sin entered the picture that the "good"-ness of creation was tainted. And sin didn't enter in through God, but through humans and spirits whom were bad.
Another way of looking at this is, "sin" is known as bad, right? And God doesn't like sin, right? So when we sin, it's bad.
If God was bad, then why would He hate something that's bad? He wouldn't. But He does. So God can't be bad, because what's bad comes from that which is bad, and what's good comes from that which is good. So if (since) God hates what's bad, then the only other option is that God must be good.
Also, God gave a way to be cleansed from the bad (repenting & accepting Jesus). So then if the bad is taken away, what's left is good. One who's bad wouldn't give a way to clean off the bad...one who's bad would want to taint the good. But God gave a way to take away the bad and make us good, and prefers the good over the bad. So then, God must be good.
picture courtesy of: http://www.jewcy.com/artists/mike_darnell