These are radar images. I think they have been through a fairly conservative vetting process before declaring these lakes. They are consistent with other findings such as atmospheric studies that have found clouds and weather associated with the ground formations. They might be able to actually see the lakes evaporate some with the change of seasons, which I would think would constitute pretty good "proof." High resolution photometry is problematic because of Titan's haze. I assume laser altimetry (MOLA) has the same problem. Over time and enough flyby's, I believe they will have enough radar detail to draw some conclusions about terrain. That was the theory to begin with and it seems valid.