NMR (Nuclear Magnetic Resonance) is a form of radio-frequency spectroscopy in strong magnetic fields. Different NMR parameters correspond to crosslink density, and carbon-black filled elastomers are transparent to radio frequency, so that NMR can be employed for determination of crosslink densities. Three-dimensional spatial resolution can be generated based on the principles of magnetic resonance imaging known in medical diagnostics, or a novel sonde can be positioned on the object, by which crosslink densities can be determined locally with limited depth resolution.