How do you tell the difference between the crops of barley and wheat?

It is difficult to tell the difference before seeds are produced, but you need to compare the auricles on the leaf collar. Wheat auricles are shorter and have small hair, while barley auricles are longer and have long hair. You would probably need a magnifying glass to spot these differences.

http://www.ehow.com/info_8417468_difference-between-barley-wheat-plants.html

With pictures! http://www.wikihow.com/Identify-Farm-Crops

Here is a manual: https://smapvex12.espaceweb.usherbrooke.ca/BBCH_STAGING_MANUAL_CEREALS_CORN.pdf

While there are many similarities (same grass family), there are differences in how the seeds arrange themselves in "spikelets"- simplifying the rather technical language in the manual, they are stacked in barley, alternating in wheat, and the long awns ("beards") extending from these spikelets tend to go upward in a close group in barley, while the wheat awns spread out more. My personal observation: at maturity, the seed heads of barley will be heavy enough to bend the stem over considerably, while wheat stems usually stay upright. Further differences at harvest: "When wheat is ready to be harvested it is in brown or golden hue and when barley is harvested it is in yellow-white hue.
The wheat has a shorter beard but barley has a longer beard." -from http://www.differencebetween.net/object/comparisons-of-food-items/difference-between-barley-and-wheat/#ixzz3WH07ibTG