Company and bank account numbers contain check digits. The last digit of a French SIREN (9 digits) or SIRET (14 digits) is computed with the Luhn formula; an IBAN has two check digits (modulo 97). A made-up or miscopied number is very unlikely to come out right.

In real life: the Annuaire des entreprises (annuaire-entreprises.data.gouv.fr) to check that a SIRET exists and who owns it, and calling the supplier on their usual number before any change of bank details. Valid check digits do not prove a number is honest; wrong ones prove it is made up or miscopied.