The easiest method is using chlorophyll meter or SPAD meter. Measures leaf chlorophyll, an important indicator of plant health. The latter performs quick measurements of the chlorophyll content of leaves without damaging the leaf.
SPAD readings are recorded by inserting the newest fully expanded leaf from each plant while avoiding old or immature leaves. Normally the fifth or sixth leaf counted from the shoot was taken where leaf mid-portion was recorded.
As
a rule of thumb SPAD reading 50-60 means that the leaves are green and healthy
while 30 and below the leaves are yellowish and not healthy.
Reports
have shown that there is high correlation between chlorophyll content and SPAD
readings.