Seperating out iron from ferrous sulfate
Decomposing ferrous sulfate to get iron(III) oxide should work:
$$\ce{2FeSO4 -&gt;[\Delta] Fe2O3 + SO2 ^ + SO3 ^}$$
Iron content can be measured from iron(III) oxide. Commercially, iron is recovered from waste ferrous sulfate in the form of magnetite ($\ce{Fe2O3}$) by co-precipitation and magnetic separation$\ce{^{[1,2]}}$. Iron content is measured from magnetite. It was observed that after processing the magnetite, the grade of iron in magnetite concentrate increased from 62.05% to 65.58% and the recovery rate of iron decreased from 85.35% to 80.35%.
References

Wang YU, Ying-lin PENG, Ya-jie ZHENG Recovery of iron from waste
ferrous sulphate by co-precipitation and magnetic separation,
Transactions of Nonferrous Metals Society of China, Volume 27, Issue
1, January 2017, Pages 211-219, DOI: https://doi.org/10.1016/S1003-6326(17)60024-4

Yu, W., Peng, Y. &amp; Zheng, Y. Effect of purification pretreatment on the recovery of magnetite from waste ferrous sulfate. Int J Miner Metall Mater 23, 891–897 (2016), DOI: https://doi.org/10.1007/s12613-016-1304-2