A. It all depends on who supplied the information and whether that person is a conscientious genealogist or someone who grabs names willy-nilly just to put more branches on the family tree.
Whether you find a genealogy of your family online or in print, the best way to judge its reliability is by first looking at the supporting documentation. Is the compiler using a healthy dose of primary sources such as censuses, military records, vital records, land deeds and the like? Does he cite mostly another person’s genealogy? Are no sources cited at all? You need to be especially cautious of the last two scenarios. If the genealogy is online, you can contact the submitter to ask for sources, but my experience has been that if a person had documentation, he or she would have included it. You can use undocumented genealogies as clues to guide your own research, but don’t put total faith in their accuracy.