Magnetic nanostructures hold tremendous potential as basic building blocks in spinelectronic devices and high-density data storage. Precise mapping of fundamental parameters like the anisotropy and switching fields over a wide range in temperature and magnetic fields, is essential to understand the influence of the relaxation, interactions and other phenomena that govern the dynamic magnetic properties in these systems. Dynamic radio-frequency transverse susceptibility (χT) experiments provide a very sensitive and unique way to probe these features. We present and discuss the field-dependent transverse susceptibility in two nanoparticle systems: (i) polymer-coated Fe particles and (ii) γ-Fe2 O3 particles synthesized by micelle method. Systematic χT scans at different fixed temperatures reveal variation of the switching and anisotropy fields. Our experiments provide a unique and powerful way to precisely probe the dynamic magnetization in the presence of thermal relaxation and interactions.