Explain how you tell if a fold was an anticline or syncline?

An Anticline has a form of an A, it means it is curved upward

A Syncline has a fomr almost like a Y, (actually is like a U), it means it is curved downward

Explanation:

Originally, sedimentary rocks are horizontal beds where the deepest bed is the oldest and the youngest bed is the surface one.

After stress by tectonics, these rock beds become curved and we can find upward and downward curves. Usually, these folds are exposed on earth's surface, then they are exposed to weathering and erosion, which make disappear part of the secuence of rock beds.

An anticline can lose it's summit, so we will find a repeated secuence where the oldest rocks are in the central part (the axe) and the youngest are in the external parts of the fold.

A syncline will show the youngest rocks in it's axe and the oldest rocks in its external parts.