The single biggest factor in finding a field or forest to hunt(that I've found so far) is research. I found an old homestead that is invisible from the road, but plainly obvious from older aerial photos. I found a reference to an entire town of 80 families in a newspaper article from 1907, the town was abandoned almost 150 years ago, to date it is unmapped, the site the town is on is now all woods. I also have the cornfield that I mentioned above. I probably have enough spots right now to last me the next 20 years, but as it's been said many times, "the treasure is in the hunt". One of my favorite pasttimes recently has been filling my "diggers journal" with new info on historical sites in my local area.
There is NO substitute for good research, it WILL narrow down the dud spots. What has worked for me is a google search for - my state>my county>history. Also try adding city names, the keywords - school, church, etc.... Many times the best places you'll pin down are smack in the middle of a field or woods. Every night that I get home I'm searching the net for new spots.
As far as hunting fields, or any spot, I usually don't worry about it unless I know there has been something there, and finding that out is easier than you might think. After all, if you're feet are the first that have ever walked there, the odds of finding anything good are worse than slim.