Is a yellow tint exclusive to thorium glass? Both Schneider Xenotars on early Rolleiflexes and the Ektar in Kodak Medalists have a definite yellow tint to the rear groups.
Yellow tinting over time is a sign that the glass may contain thorium. Thorium is a alpha emitter, which causes dislocations in the glass matrix when it interacts, Accumulation of these dislocations in the glass causes the yellow tint. All of the thorium doped lenses that I have encountered are noticeably yellow when you hold them up against a white background.
Yellow tint can also come from the balsam used to cement doublet/triplets. Canadian balsam was used because it did not yellow, but a lot of the Russian/Ukranian L39 lenses show this yellow tint, and are not radioactive. This yellow is slightly different from the thorium yellow, balsam yellow tinting is slightly brown. You'd be hard pressed to tell the difference unless you have both side by side.
A gieger counter is the best way to tell, thorium doped lenses (and radium dial watches) are easily detected thus. Most university physics departments, and radiation therapy cancer centers will have one if you want to check.
I am not aware that the Medalist's Ektar has thorium. Some early Xenotars have been reported as using thorium doped glass, but I thought they were only on larger format lenses.