Zinc wheel weights are usually marked as such...I think It,s a ZU mark...or is it ZN ?
best way to check what you have is to scratch the weight with a small screwdriver, if it takes off a curl of metal it,s a lead weight...If all it does is scuff the metal It,s zinc...
Don,t throw away the flat strip weights that stick on with tape or glue, they are usually a pure lead or pretty close to pure.....The best stuff for B/P cartridge or muzzleloaders.