How can you determine if an element is toxic?
Well elements toxicity can be determined by their reactivity.
However, it all depends on whether you mean actual elements or chemicals. If it is an element you mean, then it is determined by the reactivity by itself and if the reaction is mutagenic, or has any other adverse effect. However since its rare to find them alone, chemical toxicity is determined either by finding the LD50 (lethal dose at which 50% of animal subjects die), or they [chemists] look at the structure and determine electro-negativity and density and functional groups to determine what reaction will occur in specific environments.