Low-frequency qubit control wires require non-trivial thermal anchoring and low-pass filtering. The resulting electron temperature serves as a quality benchmark for these signal lines. In this technical note, we make use of a primary electron thermometry technique, using a Coulomb blockade thermometer, to establish the electron temperature in the millikelvin regime. The experimental four-probe measurement setup, the data analysis, and the measurement limitations are discussed in detail. We verify the results by also using another electron thermometry technique, based on a superconductor-insulator-normal metal junction. Our comparison of signal lines with QDevil's QFilter to unfiltered signal lines demonstrates that the filter significantly reduces both the rms noise and electron temperature, which is measured to be 22 1 mK.