Basically if its rich green its likely to be crop or grass after a lot of rain or irrigation. The grass isnt always green! Its a good idea to keep an eye on the local situation in regard to crop and grass colours throughout the year.
Farmers dont usually irrigate grass but they do irrigate and feed crop so hence the richer colours. Grass in fact is the great survivor it can survive drought where as in the same field crop would soon die. As the grass experiences the high levels of sunlight of summer it goes yellowy brown when not irrigated.
Other crops such as rape and wheat will take on different colouts according to the season. One of the things not very often taken into consideration are crop furrows(ploughing furrows) some of which are quite deep and will take a nosewheel off straight away if you land across them.