There are iron tests but they are not very reliable. The best way is to dose iron using a trace element fertilizer like seachem Flourish as displayed on the bottle instructions.This way you make sure that your plants have enough iron all the time. Aquaria keeping is not an exact science. Deficiencies in nutrients like iron show mostly on fast growing stem plants like Hygrophila sp. The nerve of the leaves become darker and the leaves tissue yellow pale slowly dissolving.