I just sort through 'em by hand. At first it goes really slow. Once you find a zinc wheel weight you'll know what to look for and the process speeds up some.
Here's a list of things to look for. Some things can tip you off to look closer but they aren't indicative of zinc.
Shiny: If they are dull grey, they're almost always good lead. However if they are shiny they may be OK or they may be zinc. Some good wheel weights are painted. This paint is harmless except you don't want to breath the fumes as it burns off. The zinc ones are almost always shiny. So if you see a shiny one move on to the next criteria.
Shape: All the zinc weights I've seen have had somewhat square ends. They can be oblong like any WW but the ends are squared. However, some good Pb weights have a similar shape.
Rivets: This one is the clincher. If it's rivetted to the clip, it's zinc. All the Pb weights have clips that are cast-in like the hook of a fishing jig. But my eyes would pop out of my head if I tried to look at how each weight was fastened to the clip - so I learned the other characteristics of zinc.
Sound: Zinc ones have a little ring to 'em. I grab a handful of weights out of the pale with my left hand. Then I drop a few into my right hand. Then they go from my right hand into the melting pot. If I spot anything suspicious, I toss it aside. In this process I'll sometimes hear the tinkling of the zinc.
Stick-ons: I separate all stick on weights out of the mix. These seem to be either pure lead, or zinc. I prefer the pure lead for muzzle loader balls so I don't like to mix it with wheel weights. The pure lead stick-ons are easy to distinguish from the zinc because the pure Pb can be easily twisted.
