Food Crop Research Department Knowledge
There are many signs that plants show long before wilting that, if we know how to recognize them in the early, we can really improve the overall performance of the plants and we also have a chance to start wetting and relieve the stress.
The key to getting the best results out of your crop is to know how the crop is supposed to grow. If you know what to expect at each stage, then you can have an idea when the crop is dropping back.
The following pdf document provides further insight into recognising when plants are under mild stress: Drought Stress.