Registration check. FDA Philippines runs a public verification portal at verification.fda.gov.ph where registered products can be looked up. A prescription medicine that cannot be matched there should not be used.
Carton and pen agreement. Batch or lot number and expiry date should appear on both the carton and the pen itself, and they should match. Mismatched, missing, smudged, or obviously reprinted numbering is a walk-away sign. So is a carton without a leaflet, or packaging with typographical sloppiness a pharmaceutical line would not ship.
Cold-chain plausibility. These pens require refrigerated storage before first use. A pen that arrives in an uninsulated courier bag has broken cold chain even if the pen itself is genuine, and there is no way to know how it was stored in the weeks before. This is a risk an unidentified seller cannot solve even when the product is real.
Prescription friction is a feature. The legitimate channel asks for a prescription; the counterfeit channel advertises that it does not. A seller marketing the absence of that step is announcing which channel it belongs to.