If a body is resting on a rough slope, it will be on the verge of toppling over when the weight of the body acts through the edge of the part of the body which is in contact with the slope.
Slope not steep enough for toppling to occur:
Object on verge of toppling:
Here the object will topple:
This is because, about the edge of the object, there is a net anti-clockwise moment (turning effect).
Remember, the weight acts through the centre of gravity of the object.