How can one identify an organism from its dissymmetry ratio?
It depends what lineage (bacteria, virus, eukaryotes) you are interested in and what level of accuracy you are hoping to reach.

We know the (A+T)/(G+C) ratio for a handful of eukaryotes only. So if you were thinking about eukaryotes, there is no need to look much further. You cannot match a (A+T)/(G+C) ratio to a species because we don't know the (A+T)/(G+C) for the vast majority of species.