Metal-dielectric patterned metasurfaces based on fishnet unit cells have been recently proposed as promising platforms for terahertz (THz) Fabry-Perot cavity leaky wave antennas (FPC-LWAs). Still, their experimental validation in the THz range is lacking. In this work, we design several layouts of such metasurfaces at a working frequency of 300 GHz, by varying the geometrical parameters of the unit cell. This variation allows for achieving different electromagnetic responses that in turn provide for different radiating performances when used in a Fabry-Perot cavity environment. For this purpose, we microfabricated eight different demonstrators by patterning a 200-nm aluminum film over a silica substrate through a low-cost, large-area, photolithographic process. The design flexibility of this kind of metasurfaces is then experimentally validated through THz time-domain spectroscopy (TDS) measurements in the reflection mode. The broadband analysis allowed by the used TDS techniques revealed both the simple inductive behavior and the typical dipole resonances of such kind of metasurfaces for the lower part (250–450 GHz) and the higher part (450–650 GHz) of the investigated THz spectrum, respectively. The experimental results were fully consistent with the predictions of both theoretical and full-wave analyses, thus corroborating their efficiency as a means for the design of metasurfaces with controllable THz electromagnetic response, such as the sheet impedance.