If it's dark green and you can't see dirt between rows (and maybe even if you can) it's a bad bet. If you can see the crop waving around in the wind, it's definitely a bad bet.
Cut hay stubble has a yellow tinge. Cut but not yet baled hay has a soft gray green color. You can distinguish these colors from ten miles away. Some sunglasses (e.g. "Sun Tigers") interfere with this. Avoid them.
Absent obstructions (e.g hay bales), cut hay is often a good choice. If the field won't break a mower, that's a good start.
More: A great article on off field landings from Kai Gertsen.
Happy Landings!
T8
PS: Here's how it goes in a cut cotton field in South Carolina.
