There are many factors that play into what you want to do. Again, I might have a better idea if you can post examples of your photos.
Typically, if you want to compare vegetative cover you want images taken in the infrared range of the light spectrum. The different IR signatures show up well.
If you are just looking at something using Google Maps it can be more difficult. Generally speaking, differentiating between conifers, hardwoods, and grasses is fairly straightforward. But that is also dependent on the scale and resolution of the image.
If you've got a decent quality image what you want to do shouldn't be too difficult. I'm not entirely familiar with how food plots are laid out, but if the crop is planted in rows that can usually be seen with decent resolution; but if the food plot is put down just by scattering seed then it may be difficult to tell from regular grasses unless there are areas in the photo to compare to. Typically, though, planted crops look more homogeneous where a natural prairie might have more variation as they tend to not be monocultures.