Crosses are common to most Christian groups. Mormons (not really Christian, but…) do not use crosses. Protestants on the more iconoclastic side of the spectrum might also avoid them, but that is not characteristic of Protestants as a whole. Catholics and Orthodox also use crosses of course. For examples, a "pectoral cross" is part of the outfit of Catholic bishops, and laity also commonly wear cross necklaces, so you can't say that Protestants wear crosses, but Catholics wear crucifixes. Even non-Christians not infrequently wear cross necklaces. It's probably about the most generic Christian symbol there is. A more effective way to find out his affiliation would be to ask him.