Over the weekend, temperatures dipped to high 20s/low 30s sometimes accompanied by a frost. The extent of damage to the wheat crop depends on the growth stage, temperature, and duration of the low temperature. Wheat is most sensitive to freezing temperatures at the heading and flowering stage. At heading and flowering, two hours at 30°F can result in severe yield loss due to sterility. Flowering begins near the center of the wheat spike (Feekes 10.5.1) followed by the top (Feekes 10.5.2), and bottom (Feekes 10.5.3) of the spike. Depending on the flowering stage, sterility may only occur on part of the spike.
We recommend waiting seven to ten days to assess the crop until the full extent of the damage becomes apparent. Freeze damage symptoms include white awns or white spikes, damage to the lower stem, and leaf discoloration. After freezing, the anthers are white and shriveled instead of the normal light green or yellow color.
(Source – http://www.farms.com/news/assessing-the-wheat-crop-for-freeze-damage-107566.aspx)