Here's the wording for the Tech Standards regarding disc identification. Having "PDGA Approved" on the disc is recommended but not required:
"As of July 1, 2010, all newly-approved discs must carry the approved model name either engraved in the mold or as part of the disc art, permanent sticker label or by indelible ink marking. The full model name is recommended, but the use of unique abbreviations or codes is acceptable. Such codes must either be identified with the submission for approval, or supplied to the Technical Standards Committee for variants produced after the original approval.
It is also highly recommended that all approved discs carry the words "PDGA Approved" either engraved in the mold and/or as part of the disc art or a permanent sticker. Manufacturers are also encouraged to add the model and approval identification to all previously-approved discs when it is feasible to do so. All approved discs manufactured after January 1, 2012 must comply with these standards."