Players with a farming skill of 15 or more can determine when a field is ready to harvest by activating a farming tool. The growth status of nearby tiles can be seen in the tool-tip when hovering over the tile. At farming skill 30 you can determine growth status of 1 tile further for a total of 2 tiles. (The tile you are standing on and one further)
The stages displayed (in order) are:
# Freshly sown