You should be able to see large air bubbles under the surface of the dough.
Once you're more familiar with the process - and the temperature of your home - you will be able to do overnight ferments.
You will know your dough is ready to move to the next stage when it has just doubled in size. It will be fairly wobbly and full of bubbles. You should be able to see large air bubbles under the surface of the dough.