You can tell by visually inspecting the Certificate. A masked Certificate will say “Masked Certificate of Authenticity” at the top. The Page URL will be masked and the screenshot and snapshot will be unavailable. You can also determine programmatically at the time of claim by checking the “masked” field in the JSON response. The claim response will indicate whether the Cert you are claiming is masked.
For a full overview of the concept of 'Masked Certificates' click here.