While not as unique as fingerprints, individual magmatic systems tend to have somewhat individualized geochemical profiles in terms of concentrations of trace elements and isotopic ratios so it's not "guessing" to use similarities of geochemistry to argue that two adjacent volcanoes share a source and/or have intermingling of sources in the shallow crust. Additionally, there are variety of geophysical techniques (seismic tomography, magnetotellurics, resistivity, etc) that allow us to image the magmatic systems of volcanoes which again provide evidence that these are related. The general idea that geology is all "educated guessing" (which underlies many questions and lay answers/comments in this subreddit) is pretty frustrating given the extreme care and detailed analysis many of us put into to understanding the details of natural systems.