@Ally Hey good question. I have never grown this plant before so I defer to more experienced folks when it comes to caring for it:
In terms of nutrients; I believe your best bet would be to test the pH of your soil. The magic number for your plant seems to be about 6.5 (not unlike most plants). While you can buy kits online that will help you test for various levels of various nutrients in your soil, iron is not usually one of them. Usually, if you want to test for iron you would need to either take a leaf sample and send it in to a lab or break out the chemistry kit and sulphuric acid. That being said, nutrient uptake is usually the problem (not nutrient deficiency) and the way to correct that problem is to manage your soil pH.
So my suggestion would be to follow the instructions laid out in the link I provided you for, caring for your Schefflera, and to go to a lawn and garden center to pick yourself up a pH testing kit. Just remember, lime is what you add to make the soil more alkaline and sulfur is what you add to make it more acidic.