Physiological maturity is reached when the maximum amount of dry matter has accumulated in the developing kernel and dry down of the kernel is all that remains before harvest can commence. 

At this point, the kernel transitions from the soft dough to the hard dough stage, the earliest time can you use glyphosate for pre-harvest weed control and as harvest aid.  Kernel moisture can, however, vary greatly and is not a reliable way to determine the physiological maturity. 

There are some visual indicators that are reliable and easy to use to tell whether wheat is physiologically mature, says Jochum Wiersma, University of Minnesota Extension small grains specialist, Crookston.

The green color is about to disappear from the upper portion of the peduncle. The peduncle is the upper node of the stem.

The appearance of the pigment strand along the crease of the wheat kernel. This is analogous to the black layer in corn.  To check for this pigment, cut kernels crosswise and check the color. Not all the kernels within each spikelet and across the spikelet within a head will reach physiological maturity at the same time.

Source: U of M Extension Communications