Distortion of the density of states induced by specific impurities, a mechanism known as resonant level (RL), is an efficient strategy to enhance the thermoelectric performances of metals and semiconductors. So far, experimental signatures identifying the resonant nature of an impurity have relied on the so-called Ioffe–Pisarenko plot that enables visualizing the induced thermopower enhancement at specific carrier concentrations. However, this method cannot solely discern RL from other possible band-structure-related sources of thermopower enhancement such as band-shape modifications or band convergence. An independent method of resolving this problem is proposed here. A detailed theoretical and experimental analysis of the low-temperature electrical resistivity ρ0 and carrier mobility μ0 of the resonant-level system SnTe doped with In is presented as a function of the impurity concentration x. By comparing to non-resonant cases of SnTe doped with I, Mn, and Ga, we demonstrate that the construction of residual resistivity ρ0(x) and residual mobility μ0(x) plots allows to distinguish between resonant and non-resonant impurities, even when some of them induce similar thermopower enhancements. This methodology is further confirmed by analyses performed for Na- and Tl-doped PbTe, illustrating how the combination of transport measurements at low temperatures can be used to determine the resonant nature of an impurity.