You can tell a stock is extracting flavors well by observing a deepening color, a rich and savory aroma filling the kitchen, and the bones becoming brittle and yielding gelatin.
Several indicators suggest your stock is extracting flavors effectively during the cooking process. First, observe the color of the stock. As the ingredients simmer, the stock should gradually deepen in color, indicating that the flavors and compounds are being released into the liquid. For example, a chicken stock will transition from a pale, translucent liquid to a richer, golden hue. A beef stock will become a deeper brown.
Secondly, pay close attention to the aroma. A well-extracting stock will fill your kitchen with a savory and complex fragrance. The specific aroma will depend on the ingredients used, but it should be noticeably richer and more intense as the stock simmers. This indicates that volatile flavor compounds are being released from the bones, vegetables, and herbs.
Finally, examine the bones themselves. As the stock simmers, the bones should become increasingly brittle and porous. This is a sign that collagen is breaking down and being released into the stock as gelatin. Gelatin is what gives stock its body and richness. You can test for gelatin by refrigerating a small amount of the stock; a well-made stock will gel when cold.
Don't boil your stock! A gentle simmer is key. Boiling can emulsify fats and lead to a cloudy, less flavorful stock. Maintain a low and slow simmer for optimal flavor extraction and clarity.