Magnetic Resonance Imaging (MRI) machines can generate hazardous RF heating of patients with implanted neurostimulation leads. Consequently, most patients with these implants are contraindicated from having MRI scans. The level of RF heating has a strong dependence on lead length and is most severe when the length is close to a specific resonant length. Recent studies have shown that simple modifications to the lead construction and insulating material can alter the resonant length and significantly ameliorate this heating hazard, achieving MRI safety. We propose a technique using time domain reflectometry (TDR) to find the resonant length of an arbitrary lead such to minimise the amount of MRI machine time needed to find the length of highest heating. The results are compared with temperature measurements made in a 3-Tesla MRI machine and with a CW dipole radiator in the lab.