This chart shows you if there is too much nitrogen, or you lack potassium, etc.
This pic, although it sucks, shows what insect is damaging your plants...
The same goes true for non weed plants, the same coloration and markings are on tomato plants and trees etc, so you can go into a garden or forest and see if you can spot any of these things
Now if you see plants being damaged by disease, then you see a type of plant that is not, then it has something in it that the plants don't like, so it makes a good insecticide(ferment with lactobacillus