You'd have to contact the source and ask them to provide evidence of their Boeing DPD approval. To my knowledge, there isn't a "list" of Boeing DPD approved suppliers that we suppliers can access.
There isn't much to it for a plotting source. I would create a questionnaire from Section 7.8 - and Section 2. I'd start with "If already Boeing DPD approved, please provide the current DAR." If not Boeing DPD approved, please complete the rest of the questionnaire. Either go on-site to verify, or ask them to provide documented procedures, calibration records, etc. when they return the survey.