Populations of relativistic electrons in clusters of galaxies should imprint a characteristic spectral distortion signature on the Cosmic Microwave Background photons - the relativistic Sunyaev-Zeldovich (SZ) effect. We investigate how sensitive PLANCK and successor experiments will be in detecting or constraining the total optical depth tau of relativistic electrons. We find an expected sensitivity of PLANCK of sigma_tau ~ 2 10^-5 for an individual cluster, which is too insensitive to detect even optimistic scenarios, which predict tau ~ 10^-6 to 10^-7. However, by stacking the PLANCK-signal of all SZ detectable clusters the relativistic SZ effect can be statistically probed to a sensitivity level of sigma_tau ~ 10^-7. PLANCK successor experiments will be able to probe even conservative scenarios of relativistic electron populations in clusters of galaxies.