John Shriver , Jul 23, 2007; 10:27 p.m.
One obvious hint is any paper in contact with it over time. Platinum is a strong catalyst, often paper facing a platinum print has brown aging marks from the print.
Any sign of a baryta layer would rule out platinum.
You would be looking for a matte surface, a soft grey-black image color.
They got impractically expensive about 1890.