There are some visual indicators that show when wheat has reached physiological maturity, says Jochum Wiersma University of Minnesota small grains specialist
The first indicator is the color of the upper portion of the peduncle. The peduncle is the upper node of the stem. Just below the rachis the peduncle has a characteristic bend.
When the green color is about to disappear from this bend, the crop has reached physiological maturity.
A second indicator is the appearance of pigment strand along the crease of the wheat kernel. This is analogous of the black layer in corn. To check for this pigment, simply cut kernel in crosswise and check for the pigment strand. Understand that not all the kernels within each spikelet and across the spikelet within a head will reach physiological maturity at the same time.