Finding the selector
Open a message sent by the domain, show original headers, and look for DKIM-Signature. The s= tag is the selector; d= is the signing domain. Common values: google (Workspace), selector1 / selector2 (Microsoft 365), k1 (Mailchimp), plus whatever your ESP documents.
A healthy record starts with v=DKIM1 and includes p= followed by a base64 public key. An empty p= revokes a key. Guide: DKIM explained.