Um, a way to go about this...

Give the "boulder" a rigid body... And then add a collider that's attatched to it, so that when you move it, it only moves because of the physics, and stops when it hits the collider... Could be easy...

Alt: You can animate this... if player goes to spot A, and pushes F, then boulder rotates and moves ->>> if that makes sense.

You can use a distance check.

You need a few varaibles:

Vector3 targetPoint - The target in world space.

float actionDistance - How far the boulder needs to be from targetPoint before triggering the action. If you want the boulder to have to be in a specific spot set this equal to the boulder's radius

float radius - The radius of the boulder.

Here is some code

public void Move(Vector3 pushForce) {
        // First, update our position based on the push applyed by the player
        transform.position += playerPushForce * Time.deltaTime;
        // Find a vector going from the center of this boulder to the target point 
        Vector3 difference = targetPoint - transform.position;
        // If the distance is less than the actionDistance - radius, then we have a hit
        // (the dot product of a vector with its self is the squared magnitude of said vector)
        if (Vector3.Dot(difference, difference) < (actionDistance * actionDistance) - (radius * radius)) {
            // Hit it!
        }
    }