You have a heart face shape if: you are wider at your forehead then narrows below your cheekbones and your chin is defined and pointed.

Facial Features:

Wider at the forehead
Usually defined cheekbones
Tapered narrow chin 

You have a square face if: the sides of your face are straight and your face is as wide as it is long.

You have a defined square jawline and your hairline is probably straight. 

Facial Features:

Square shaped forehead, cheekbones and jaw line are the same in width.

A squared jaw line.