Here is an interesting article detailing the history of forensic testing, specifically for arsenic. Budding forensic scientists could detect arsenic, in a body, by the 1840’s. One test, the Marsh test, became the go to test well into the twentieth century. The true value of the Marsh Test was the process used to validate the results, in court, became the blueprint for admitting expert witness testimony. In the end, James Marsh did more than put arsenic poisoning decisively out of fashion: he created the lineage of modern forensic science in court. Marsh-test-arsenic-poisoning.
If one were to read early detective stories, up through the 1930’s, poisoning was near the top of the list as a murder weapon. Modern fiction rarely dwells on poisoning as a method for getting rid of a pesky individual. Poisoning only comes into play when used on an industrial scale, to take out entire towns.
Oh, for simpler days.