I use canaries. I point a dozen domains to fastmail and another dozen to my self hosted email servers. Each have aliases that are mapped to vendors but do not have the vendor name as some vendors are getting upset at this practice and calling it fraud. If I start getting garbage on that alias, I notify the vendor. In most cases they will give me a boiler plate response and then I delete the alias. If they are snarky I create a reject rule with my own snark that also explains the emails for that vendor have been either sold or compromised. This is to let people buying email addresses know they bought a dirty list as some of the modern bots have some telemetry.