Nature, it would seem, has a way of speaking to us.  Of course clouds and humidity give hints at rain, and leaves falling tell us of the imminent autumn.  But horticulture as well gives clues as to harvesting and readiness. 

Notice how the onions in the middle and left are laying down their green stems?  This is process called "flagging", whereby the onions signal that they have developed enough carbohydrates over the season and are finished expanding their bulbs.  It is only after this critical step that a farmer (like Jack) would know they can be harvested and cured.  

The white bulbs here are in a "pre-curing" phase, laying out in the sun to dry.  The key is to get the green stems to dry out, which now serve as a straw-like function to get the onions the proper nutrients, water, and light.  Once harvested though, the stems need to dry to be able to eliminate the straw feature, which could at this point let unwanted diseases enter in, prompting the onion to rot.   

Melons also have a convenient way of telling farmers they are ready to be plucked from the field.  The tendril that is closest to the fruit is still a green-yellow.  When it turns a beige, dry color, it communicates it has stopped growing and is ready for harvest. 

Cantaloupe too, while they don't have that specific tendril, tell of their readiness by their stem closest to the fruit being easily plucked, so that the node base of the stem releases nicely from the melon.