If we find DNA based microbial life it could certainly be amazing evidence in favor of transpermia. We would at least compare to DNA on earth and analyze similarities. Even plant and animal DNA are very similar (double helix shape)
Actually the easiest way to find life is to just look for DNA (or even better RNA) in the environment of interest. This is basically the way life is found in some of the amazing environments here on earth like the "lost city [0] or far under the earth [1].
Because of the technical simplicity and sensitivity of looking for DNA, any probe sent to any planet or moon would look for DNA first, basically assuming transpermia is true.
