If I'm walking through and there is a large flat area where it looks like something has been cut down I take priority to it. Area's near rocky walls and mountains also tend to be hot spots. Otherwise I just go off the beaten path everywhere and sometimes go by what I find as to what else could be in there. Unless you see a whole old structure in an area it's hard to tell by eye right away. Though it sucks that the best spots are usually the most rocky and difficult unless you hit a jackpot in a very clean, easy digging area which has yet to happen to me.