If you have access to a Magnetic Susceptibility balance, it should be fairly easy; as the ferric and ferrous might have a different number of unpaired electrons, so the reading should be quite distinct. No guarantee, depending on the binding and what little info you provided, but still worth a try.
You would have to create some standards, but that should not be to hard.
While they are not cheap enough for anybody to buy, as far as good chemistry equipment goes, the magnetic susceptibility balance is relatively cheap; and always under used in my experience. You should ask around your chemistry department, someone might have one, even the organic chemist and some inorganic chemist should have one!
The bonus is it takes very little of the sample, and you do not alter the sample in anyway and as a result you can reuse it for whatever else you are doing. It is also incredibly quick.