So, this is what Seachem's response is to my question.
"impossible to test for the the iron quick enough.
It is easy to test for other iron additives since they are in the ferric state. Plants have to first convert iron from the ferric state to the ferrous state before being able to utilize the element properly. This conversion does take time, therefore a hobbyist will be able to get a reading for iron after dosing ferric iron.
Our FAQ below explains this further:
Q: I've got a 40 gal heavily planted tank with Flourite substrate. I started dosing with Flourish Iron a few weeks ago due to pale leaves on pennywort, etc. The color has improved. I'm adding a capful every day to the tank; an hour after adding, my tank shows no iron with the Hagen test kit. Are the plants grabbing the iron? Or is it precipitating out somehow.
A: I am confident that the iron is being consumed. If it was precipitating out of solution, you would notice a white chalky haze to your water and a white colored coating to your aquarium walls, heater, filter, etc.
Our Flourish Iron is a very different type of iron. Our's is bound to a carbohydrate that is very easily metabolized by your plants. Most of the others on the market are chelated iron supplements and your plants has to expend large amounts of energy to get the iron from the chelate to be able to use it."
And this is what I found by doing a search for ferric
"Plants must reduce (ferric iron) Fe3+ to (Ferrous iron) Fe2+ before the iron is available for absorption, so providing iron in the ferrous form should be more efficient. While the difference may or may not be important to the aquarium hobbyist, I wanted to clarify the distinction to help the user to make that decision."