Bolt head corrosion in soup. Toxic? Zinc? Cadmium?
You can test for cadmium at home, but the ~US$30 price for the test kit (with 1,5-diphenylcarbazone) is probably more than the soup is worth, and the test might not be sensitive enough to detect sub-acute toxic amounts of Cd.

Considering the highly toxic nature of the metal, you might check with your family physician if you and anyone else sampling the soup should have a blood or urine test.

As has been mentioned, the small amount of zinc that might have been absorbed had the bolt been galvanized (unlikely, anyway) is not significant. The US NIH recommends ~10 mg of Zn in the diet per day as a nutrient.