The custom of clinking glasses before drinking a toast was originally based on discovering that what you were about to drink was not lethal.
At least it’s one of the leading theories behind glass clinking. This theory considers that at the time of the Roman Empire, poisoned beverages were the modus operandi for assassinating unwanted political or economic rivals. By the Middle Ages, a kind of “poison epidemic” had grown, stimulated by the increased availability of medicinally-suspect ingredients from apothecaries throughout Europe. At about the same time, Arabs developed an odorless and transparent form of arsenic; woe to the Crusaders. Thus, self-preservation dictated that a visitor would insist his host drink some of what was poured for the visitor himself. Once the host drank “successfully,” the guest could imbibe with confidence. Eventually, in situations when the guest trusted the host, the process was done away with and supplanted by clinking—an acknowledgement that all was safe.