Observe for a month. The only exception is doing praziquantel treatment for fluke infestations. Unless you have a microscope, then there is no way to tell if your fish has flukes other than the scratching symptom which could still be associated with other health issues. Prazi is a mild treatment but you must not abuse its use since there is a potential risk of developing resistant strains of flagellates and flukes to prazi which may become difficult to treat later on. Treat only once for a week or two while under quarantine.
Other essentials I keep for emergencies are trichlorfon for fish lice and other external parasites, and then salt for ich. I keep goldfish myself so I have to be prepared especially for those that came from ponds.