Use a casting thermometer. If the melt still looks soupy at 800 it is probably contaminated with zinc.
When melting I keep temps at 750 or less with clip on wheel weights in the event a zinc weight makes its way into the melt it will float up with the clips, and steel weights. For the stick on ones I usualy have to get them up to around 800 degrees to get them to pour well.
Note make sure you melt weights outside. It releases Carbon Monoxide to melt them inside, also it makes the house smell like burned tires for days afterwards.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.