You cant exactly.(most of the time) When your experienced,you can estimate,and you often get it pretty close.For example, i recently completed a garden job, " how many do you think youll catch " asked the householder.Based on the activity levels,and the size of the area,i said between 6 to 12 moles.Turned out i got 12 ,her man cleared the hills,and it was clear for three weeks.Then a few hills were heaved up again, i got two more.This was new moles moving into the cleared territory.