The standard is able to be assessed for conformity via a ‘first party’, ‘second party’ or ‘third party’ system.
The term 'conformity assessed' should be used for any claims of compliance.
The term 'certified' can be used if an 'accredited certifying body' undertakes the assessment. Certifying bodies are able to be 'accredited' once an 'accreditation scheme' is developed for ISO 20121. At the time of writing (early 2014) there was no accreditation scheme in place. Therefore all claims of being 'certified' are misleading.
Be sure to use the phrase 'conformity assessed'.