The conjugation of magnetic nano- and microparticles with biomolecules and its analysis very often require a dense coating of the iron oxide to prevent any reaction with redox-sensitive molecules or ions. The shielding of the iron oxide cores in different types of magnetic particles was compared by rapid assays that are based on the analysis of redox reactions. The oxidation of cysteine to the corresponding cysteine disulfide in the presence of incompletely covered iron oxide was analyzed with Ellman's reagent. Similar results were obtained by analysis of the reduction of copper (II) to copper(I) by the magnetite containing particles under the conditions of the Bicinchoninic Acid (BCA) assay. We show the assay results for different commercially available magnetic particles in the size range of 20 nm to 30 μm and conclude that an incomplete coverage of the iron oxide cores requires the comparison with reference particles in redox-sensitive assays, e.g., for the characterization of the protein coating or of the density of functional groups on the particle surface.