If I am given the dissymmetry ratio of an organism ((A+T)/(G+C) is a constant), how can I identify the source of the DNA and classify the organism?
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.
You could compare the (A+T)/(G+C) to those of higher orders; see "applications in systematics" on https://en.wikipedia.org/wiki/GC-content
( While the (A+T)/(G+C) ratio doesn't provide enough resolution to identify the species, OP didn't ask for identification of species.)