As for any valuable philatelic item a certificate from a recognized authority is recommended. APEX or Philatelic Foundation are recommended. I am partial to APEX for postal stationery items. Details for submission to APEX are available here:
https://stamps.org/services/stamp-authentication.Per the UPSS catalog UX27D is a recut of the regular UX27 and UX27D only occurs in dark green on grayish white card stock. Measurements are Height 24mm, Inscription Length 58mm, Overall Design Length 97.0-97.5 mm. Paper is horizontally fibered. Stamp impression is distinguished by heavy hair lines with que and coat lines distinct and coarse.
There are no known counterfeits or fakes of this card but there are misidentifications of the normal UX27 based on paper. Of course the openness of the que is a typical giveaway as to the identity.