What kind of a "field" is it - agricultural crop land, or a natural, undisturbed setting?
If it is in a hay or stubble field (corn or soybean)....or even if not combined yet, set both the highest ground and at the lower areas on the field edges where the woods or a lane might intersect - all natural travel ways. Sign may not always be that evident, but if you know that you have fox in that territory set the corners/edges and the high ground.
Your effort will improve with practice, trial and error, and cultivating a keen sense of observation, learning how to find, interpret and translate the sign in any given habitat.
The examples below are prime red fox habitat where I trap with edges, lanes, high spots and wooded points into fields.
Jonathan
Soybean Field - Not Picked; Set At Tree Line And Crop Corners

Corn Field

Open Hay Field - Wooded Point

Old Hay Field Edge - Red And Grey Fox Habitat

Manure Pile In Hay Stubble

Wooded Point In Disked Corn Stubble

High Point Between Hay Stubble, Lane And Worked Field

Wooded Island In Disked Field

Corn Stubble And Wooded Point