How can I tell if a 'new' unit is actually refurbished or open-box?

Check before you accept delivery, not after, and put the burden on the seller in writing before that.

The physical checks. Factory seal intact: brands use tamper-evident tape, and a carton with a second layer of ordinary brown tape over a cut seal has been opened. Accessories packed the way the factory packs them, which is very precisely: remote still in its sleeve, cables tied with the factory tie, manuals flat, microphone in its bag. Refurbishers rarely reproduce that fussiness. Serial number on the carton matches the one on the unit. No dust in the vents, no fingerprints on the fascia, no rack-mount or foot marks on the underside, no smell of warm electronics from a box that is supposed to be cold.

After setup, one more. Many AVRs show total usage hours somewhere in the service or information menu; where it exists, a new unit should not have dozens of hours on it. Menu availability varies by model, so treat it as a bonus rather than a rule.

Does an open-box unit sound worse? Usually not. That is not the point. An open-box or refurbished unit may be sold legitimately, but it should be sold as one, priced as one, and covered by whatever warranty actually applies to it. The problem is a used unit priced as new with a "new" warranty that the distributor will not honour once the serial's history is checked.

The most important protection is not on the carton at all. Buy from a seller who states in writing, before shipping, that the unit is new, sealed, current Indian stock. That one line, in a message you keep, is worth more than every inspection above, because it converts a suspicion into a claim.

If something looks wrong at the door, refuse delivery and message the seller with photos then and there. Once the box is signed for and unpacked, the argument gets harder. We ship sealed stock and we will put that in writing on request, so you have the line before the courier rings.