you can never accuratly judge how deep a bury is going to be by looking at it, but as a rule burrows in flat open fields tend to be shallower, burrows are dug at an angle into the ground so burrows on steep hills tend to go deep because the rabbits dig downwards into the hill and it is difficult to get on top of the ferret to locate it without ending up facing an 8ft dig.
on one of my farms there is a series of flat open fields and on the other side of a stream there is a big hilly moor type peice of land but i dont want to risk ferreting most of the warrens on the moor so im leaving that