The motion of quantized vortices is responsible for many intriguing phenomena in quantum-fluid systems. A central challenge in modeling vortex dynamics is to evaluate the dissipative force caused by thermal quasiparticles scattering off vortex cores. Various models have been proposed, but it remains unclear which model describes reality due to the lack of comparative experimental data. Here we report an unprecedented visualization study of quantized vortex rings propagating in superfluid helium. By examining how the rings spontaneously decay, we provide decisive data to identify the model that best reproduces observations. This work eliminates long-standing ambiguities about the dissipative force on vortices, which will have a far-reaching impact since similar forces have been adopted for other quantum fluids such as neutron stars and gravity-mapped holographic superfluid.