Flat-band (FB) systems offer a fertile platform for realizing exotic correlated and topological phases, yet central challenges remain in tuning FBs towards the Fermi level EF and in understanding their impact on electronic transport. Identifying how close a FB lies to EF normally requires advanced spectroscopic probes or extensive doping studies hindering fast screening of promising candidates. Here, we show that standard electrical resistivity measurements already contain a clear and universal fingerprint of the proximity of a FB to EF: A characteristic curvature in the temperature-dependent resistivity at high temperatures, i.e., sublinear (superlinear) behavior when EF lies within (away from) the FB states. Within a phenomenological model, we can attribute this response to the rapid change in scattering phase space associated with interband transitions from dispersive to FB states. Notably, this characteristic emerges independently of microscopic details, such as the electron-phonon coupling strength, as confirmed by comparison with ab initio electron-phonon transport calculations. We validate this mechanism across several FB materials-including kagome, pyrochlore and Cu₃Au-type intermetallics-by combining high-temperature resistivity measurements with a modified Bloch-Grüneisen analysis. Our results establish a simple and broadly applicable method to locate FBs via electronic transport, enabling rapid screening of promising FB materials.