Good suggestions at the bottom.

There are several indications which could be used to discover the attack from day 1:

All issued SSL/TLS certificates are subject to certificate transparency. It is worth configuring certificate transparency monitoring, such as Cert Spotter (source on github), which will notify you by email of new certificates issued for your domain names

Limit validation methods and set exact account identifier which could issue new certificates with Certification Authority Authorization (CAA) Record Extensions for Account URI and Automatic Certificate Management Environment (ACME) Method Binding (RFC 8657) to prevent certificate issue for your domain using other certificate authorities, ACME accounts or validation methods