How do I know if my vitamin C serum has gone bad?

L-ascorbic acid formulas oxidize over time and turn from clear/pale yellow to dark orange or brown. Once that happens, the active is degraded and the serum can actually irritate or stain skin. Buy smaller bottles, store away from light and heat, and replace every 2–3 months after opening. Derivative-based serums are more stable and generally have longer usable shelf lives.