There are some clothiers that mass-produce their garments. I wonder if there is
a) a list of companies that don't use Sha'atnez
b) a list of mass-produced garments (such as "Classic Blazer" by Joe Schmoe) that are never made with Sha'atnez due to the designer/manufacturer's specifications for that garment, or
c) any Halachic principle (Hazakah, Rov, etc.) that can be relied upon if there is no such official list as above (ie., nobody checks manufacturers, either by phone/written inquiry or by visiting), but enough garments have been checked and are known to Sha'atnez checkers not to have Sha'atnez in them.