There are a few tell-tale signs that can help indicate when it’s time to get your loaf baking. First of all, when you peek through the window in your oven to check the progress, you should notice that the dough is about double the size of when you placed it in the oven to proof (via Food Network).
Since it may be tough to determine exactly when the dough has doubled in volume, you can also test it with a simple poke. As Bon Appétit advises, give a hardy poke to your loaf with your floured or oiled finger or knuckle, and then watch. If it immediately springs back, it needs a bit more proofing time, but if it’s a bit slower to recover, you can get that loaf baking ASAP (via Bon Appétit). If the indentation stays and the dough doesn’t bounce back at all, you’ve likely over-proofed your loaf (via Modernist Cuisine).
All hope isn’t lost if you’ve let your carefully kneaded loaf hang out a bit too long in your steamy oven, though. During the proofing process, the reason you see that rise in your dough is that the yeast cells within it release carbon dioxide gas (via MasterClass). According to Modernist Cuisine, on most loaves of bread (sourdough is a bit trickier) you can reverse the proofing a bit by pressing firmly to release the gas and then reshaping the loaf to try again.