Quick notes from Wet Web Media
The principal selection criteria for picking out healthy dragonets are their fullness of body and activity level. Suitable specimens should not be skinny, and should be out and about, investigating their environment. For sure you want to see the specimen/s eat.
Disease: Dragonets are notably slimy fishes that are not as susceptible to external parasite infestations as other fishes. [For this reason, and because the typical quarantine setup lacks the live food necessary to sustain these animals, the quarantine of visibly healthy specimens is best skipped altogether in favor of an extended pH adjusted freshwater dip/bath.] However, they are not immune to disease/infection, and are overly sensitive to copper compounds, other metal-based and formalin containing medications. They are best treated through environmental manipulation (hyposalinity, elevated temperature) should they show signs of such afflictions.