How do I know my respirator actually seals?

OSHA-style fit testing is the only definitive answer, but at home you can do a user seal check: cover the exhalation valve and exhale gently (face should slightly pressurize without leaks), then cover the cartridges and inhale (mask should collapse slightly toward your face). Specialist community guidance we read emphasizes that fit varies by face shape, so a mask that fits one reviewer may leak on you regardless of brand reputation.