Usually when in a crime, we confirm a stain or a sample with respect to DNA, fingerprints, etc. But in the worst case scenario, when there is no finger print evidence and both the blood samples are of same DNA, the this method called "Antibiotic Profiling" comes into picture.
The main principle is that antibiotic resistance is not an absolute property. Even within a single species, strains can exhibit different degrees of antibiotic resistance, humans usually have some varying degrees of resistance to the types and concentrations of antibiotics used and the development of antibiotic resistance profiles or “fingerprints” makes identification of source. Methods used as, in short, primarily: Colony Forming Units counts, Respective Bacterial Population Profile method, etc, and then by gel electrophoresis for segregation under electric medium.