In this theoretical work, we present a spectroscopic analysis of the cis/trans-isomers of a molecular switch, penta-2,4-dienoic acid, attached to gold clusters of different size (1, 2 and 20 gold atoms). We have simulated 4 different spectroscopic techniques: Infrared spectroscopy, normal Raman scattering, absorption spectra and resonance Raman scattering. We discuss how the position and the conformation of the molecule determine the electronic structure and hence, the spectra. The calculations have been performed using density functional theory for the properties of the ground state and time-dependent density functional theory for the excited-state properties. Special emphasis is put on the resonance Raman spectra for the study of the isomers. In the present case, resonance Raman scattering is best suited to discriminate between the isomers on the gold clusters.