go to a good bikeshop whose employees actually ride mountainbikes, buy a GU (or something small, but buy something), and ask for trail info.

word of mouth and local knowledge is what is mainly used.

or explore. seriously. get out there and just ride around and look, sometimes you find really really cool stuff. and even when you don't, you have a fun adventure.

wherever there are powerlines, there are at least large dirt trails...start there.