The notion that all poisons leave a visible trace is a fallacy. Instead, a visual examination is almost certain to not yield any conclusive evidence.

Therefore, a forensic toxicologist needs to have an eye for detail, especially at the crime scene where items such as chemical residues, pill bottles and powders may aid them in narrowing down the possible causes.

In terms of a body, living or dead, a thorough examination of sympoms would be recorded.

With both the above considered, the forensic toxicologist would then determine which toxic substances are in evidence, their amounts and, also, their effects.

Analysis of substances already within a body can be extremely difficult due to the body manipulating them. For instance, many restricted drugs alter as they are metabolised, i.e. heroin becomes morphine.