By the spacing of the stones.
They will dig a large trench from the edge of the cove to the edge of the field.
If they find burning pits or socket holes then the avenue went further.
Geofiz didn't show up much but then it never really does.
Last year they tried to locate the stones that stukeley drew near the longbarrow but didn't find anything.
However I found them :)
They are in the garden of the stables. A local jockey told me they would have been moved there as longstones field was cleared so that the race horses could use the field.