If: The dough pops back out quickly – This means its under-proofed. The dough stays where it is – This means its over-proofed. The dough pops back out slowly and leaves a slight indentation – Perfect, your dough is ready!
Pinch off a piece of dough from your sourdough before you place it into it's fermentation container. Place the small piece of dough into a small shot glass. It will be easy to see when the dough in the glass has doubled. When this happens, the dough in your container should have also finished its fermentation.