Light meter readings. Plant tissue analysis results, e.g. sap readings. Emitter flow rates and uniformity measurements. Fertilizer stock recipes and any adjustments. Volume of leach tank pump out.
Experienced growers know what healthy plants look like.
But, be careful here that you don't associate green, vigorous plants with higher yield. Knowing what kind of plant will produce the best fruit with the least trouble is the most important key to being a successful grower. This means that you must devote a portion of each day to being a plant reader and record taker. You will be glad you did when it comes time to diagnose a problem. Read your book from last year before you start up the new season, and read the old book periodically through the new season to see what the problem was last year and what you did to correct it.