How can I understand that e-mail under my domain is not validated?

In the "Manage your account" panel the domain has "E-mail not validated" status, which can be viewed:

• in "Review and update data" section (in "Services" menu),
• in "My domains" section (in "Services" menu),
• On a domain page ("Services" menu  →  "My domains" →  domain link).

If no e-mail under domain is validated, whois contact (Registrant type) will also have "E-mail not validated" status.