Visual Deficiency Indicators: 

Calcium: woody stems and cell death at base of leaves

Nitrogen and/or Sulfur: pale yellow-green color leaves

Phosphorus: purple color on lower leaves and/or overall stunted growth

Potassium and/or Magnesium: yellowing and cell-death around edges of leaves or in between veins

Brix: measure of dissolved solids in plant juices, including sucrose and fructose, amino acids, minerals. This measurement is found using a refractometer. A refractometer costs about $100.

Use for brix testing: get a brix reading on an individual plant as a baseline, apply a foliar feeding of a supplement/fertilizer to the plant, and take a second reading an thirty to sixty minutes later.  Based on whether or not the brix number rises, judge whether that supplement or fertilizer is in fact useful to the plant (and the rest of that crop) at this particular time.  

Dr. Carey Reams: did original brix research 

Rex Harrill: has a brix website

Ideal brix reading is often minimum 12, though this varies for different plant species.

Plants with a high brix reading have low to zero pest problems. Vegetables/Fruits with higher brix count taste better, last better, and are way more nutritious.  High brix fruits and vegetables don't rot; they virtually dehydrate.  Carey Reams brought the same individual watermelon to the state fair three years in a row to prove that his high brix specimen had lasted three whole years.

Sap pH: Ideal sap pH is 6.4 (above means shortage of critical anions: nitrogen, phosphate, sulfur; the higher the pH the worse the problem.  Below 6.4 means shortage of calcium, magnesium, potassium, or sodium). Also! Urine and saliva in people should be 6.4.

Baking soda can help curb certain fungal diseases, merely by raising plant pH

Sap pH is an expensive test to get done.