How can you tell the type of weathering that caused a cliff to wear away?

Dissolved minerals indicate chemical weathering, such as by acid rain. The presence of rusty red soils would also be an indicator of chemical weathering by oxidation. Cracks and broken rocks indicate physical (mechanical) weathering by ice wedging or animals burrowing. Smooth, rounded surfaces indicate mechanical weathering by an abrasive force such as sandblasting.

Weathering is the process by which larger rocks are broken down into smaller pieces over time. Weathering may occur either mechanically or chemically. Mechanical weathering occurs when a rock breaks down into smaller particles without changing composition. Mechanical weathering involves some sort of physical force. Ice wedging, exfoliation, burrowing done by animals, and root wedging are agents of mechanical weathering. If a rock is broken apart into smaller pieces via chemical wreathing, then the composition of the rocks changes. Chemical weathering can be caused by acid rain or oxidation.

This link provides a detailed explanation of factors that cause weathering to occur.