Using evidence rather than just "damn that looks like a bishop to me". There's lots of other old chess sets to compare to, and it sounds like these are from 11th/12th century which would be hundreds of years before bishops are thought to have been added to chess.
A. Some random folks on the internet who have looked at a single picture of a chess piece and maybe play themselves, but apparently no other qualifications.
Or:
B. Medieval historians who have years of education and study, deep familiarity with the subject matter and publish books and papers on the subject, as well as consulting with historians of chess, and who have compared it to other well known, well studied chess sets from the era.
I don't think the latter are infallible, but without something pretty compelling from the former (evidence? what's that?), I'm gonna go with the latter.
