One trick I learned years ago - this was passed down from an old time trapper that actually trapped for a pay check in the winter months due to seasonal layoffs - This is for fox and coyote mostly -

When trying to find a set location in an area that you know holds fox and coyotes - go out to the field squat down and look around - find the first thing that catches your eye - A lone tree, a dip or hump in the land scape, farm machinery, a rock what ever it is walk over to that spot and do the same thing from that location - it usually ends up being a clump of grass a rock or mound of dirt from plowing - go to that spot and make your self a dirt hole set. This guy swore by this strategy and it seems to work well for me too.