You can distinguish a porcelain sink from an enamel sink by testing its composition. A porcelain sink is typically solid with no frame underneath, an enamel sink will have a frame usually made of steel or iron – as a result, a porcelain sink will be much lighter than enamel.
Visually, the two sinks are very similar. To test for an enamel sink, use a strong magnet and see if it sticks. Due to their metal frames, the magnet will stay put for an enamel sink but fall straight off of a porcelain sink.