State board of pharmacy license
What it measures: Current, active pharmacy license in the state the pharmacy operates from, and separate non-resident licenses in every state they ship to.
Where to look: Look up the pharmacy by name on the state board of pharmacy website for the state their street address sits in. Every state has an online license lookup. Then repeat for your own state if it is different.
Red flags: License lapsed, suspended, disciplinary order on file, or no license listed in the state you live in.