Telling a nutrient deficiency from leaves is quite easy with some of the tables of ID pictures around.Old leaves showing symptoms,young leaves showing symptoms,yellow veins or interveinal yellowing, general yellowing,bunching small or yellow veins etc can let you pick which nutrients are deficient.Secondary deficiencies induced by excess of one nutrient in fertilizer and multiple deficiencies are hard to pick. Excesses and toxicities can also be hard to identify and seperate from some diseases.Excess chlorides burn leaf edges and especially tips and excess Na bronzes leaves and these are easy excesses to identify..