There are several physical signs that indicate whether a hen is productive.
Comb and wattle: The color and size of the comb and wattle indicate the activity of the ovaries. The reproductive organs increase in size before the start of laying, and this increase is accompanied by an enlargement of the comb and wattles.
Figure 1. Comparison of the comb and wattles of a productive (left) and non-productive (right) Single Comb White Leghorn hens. Source: Jacquie Jacob, University of Kentucky
Skin color: If the breed of chicken in question has yellow skin—for example, Leghorn, Plymouth Rock, or Rhode Island red—the intensity of the yellow pigment in the skin and scales indicates the level of production. The darker her skin pigment, the fewer eggs a hen has laid.
Figure 2. Comparison of the skin color of poor (left), medium (middle) and good (right) egg producers. Source: Jacquie Jacob, University of Kentucky
Abdominal capacity: Standard-size hens that are currently in production typically have a large abdominal capacity. To evaluate the current level of production, you can count the number of fingers that fit between a hen's pubic bones and between her pubic bones and the tip of the keel. This measurement gives you an idea of abdominal capacity. A large abdominal capacity is equivalent to about three fingers by three fingers.
Figure 3. Examples of poor (top), good (middle) and excellent (bottom) abdominal capacity. Source: Jacquie Jacob, University of Kentucky
Abdomen fat: Hens that are in production have very little fat in the abdomen, a condition indicated by increased pliability of the skin in the abdominal area.