The gap between a trading desk and a real factory shows up in exactly five verifiable places.
1. Factory evidence. Ask for equipment photos with date stamps, ISO 9001 / IATF 16949 certificates with matching company names, and — most telling — a short video call walking the floor. Factories accept; middlemen deflect.
2. Engineering response. Send a drawing with a deliberate ambiguity (a missing tolerance, an unclear material callout). A capable manufacturer asks questions and flags risks; a reseller quotes anyway.
3. Sampling policy. Transparent mold and sample fees — refundable against volume — plus a clear 7-12 day sampling lead time. Vague free-sample promises on custom parts usually mean your project is being resold to a third party.
4. Quality documentation. Ask what arrives with samples: dimensional report, material data sheet, hardness reading. This predicts what mass production will look like.
5. Export track record. Which markets, which ports, familiarity with your shipping terms and packaging standards.
Rubbrother passes all five by design: 10 compression presses on-site in Dongguan, ISO 9001/14001 and IATF 16949, AI-assisted quoting within 24 hours, and documented sampling with fee-refund policy.