How do we know there must be a force on a falling apple?

If we put a block on a smooth table
it keep at rest
if we apply a force on it , it start moving ....(1)
but it travel in constant velocity only
to keep it's velocity increase , we must apply a continous force on it ...(2)

now consider the apple
initially it is at rest
if it fall down , it must be caused by a downward force (by (1))
and it keep increasing velocity for a certain distance
this shows that the force is continous (by (2))

using this formula
F=ma (force = mass x acceleration
which means when there is force acting on an object(apple) it must either accelerate, decelerate or at constant speed
before the apple fall, gravitational force is acting on the apple. when you let go/ when it start falling from the tree, the apple would go from no velocity to accelerating towards the floor. it is the acceleration motion told us that there must be a force acting on a falling apple/ object

hope this can help you( =]