Under the GDPR, certification is conducted by national certification bodies or by the competent national data protection authorities (Art. 42(5) GDPR).
For further information, we recommend contacting the relevant national DPA for your organisation. You can find a overview of all EEA DPAs here.
You can find further information regarding certification in the EDPB guidelines on the topic: Guidelines 1/2018 on certification and identifying certification criteria in accordance with Articles 42 and 43 of the Regulation - version adopted after public consultation