1+ to what everyone else has said. They'll be the last to melt and will float on top of the liquid for quite a while before melting. If you start out small with the melt and just add a few clips along and skim often you'll see which ones are hard to melt and can be skimmed off.
Many look different than the lead ones and have rivits but not all. They are harder than the lead ones and won't score as easily with a knife or side cut pliers. Another test is dropping them on concrete. Zinc weight will bounce way higher and have a "ring" to them when dropped much more so than lead alloyed WW.