My friends father works for the DEA. He flies around in a helicopter looking for marijuana plants. Apparently they're very easy to spot. Not only are they shaped unlike any other plants when looked at from above (like a star) they are glistening in the sunlight because of all the crystals formed from the THC. He said they literally jump out at you because no other plant reflects light like this. Hemp doesn't produce these crystals, so no, you can't hide marijuana in a hemp field.
"The world is a dynamic mess of jiggling things, if you look at it right." - Richard Feynman