If you already have a Trezor from a non-official seller, these are the checks from Trezor's own docs:
Every Trezor ships without firmware. If a new one says firmware is already installed, stop and contact Trezor support.
Safe 3 / Safe 5 / Safe 7: Trezor Suite runs a Secure Element authenticity check during setup (a challenge signed with a factory certificate). You can rerun it under Settings > Device. Don't turn it off.
The bootloader only runs firmware signed by SatoshiLabs, and Suite flags firmware revisions that don't match.
There is no default seed. A device that arrives with a pre-filled recovery card is a scam.
Caveat: Ledger Donjon showed the Safe 3 authenticity and firmware hash checks can be bypassed with voltage glitching, so the checks don't replace buying direct.
If you already have a Trezor from a non-official seller, these are the checks from Trezor's own docs:
Caveat: Ledger Donjon showed the Safe 3 authenticity and firmware hash checks can be bypassed with voltage glitching, so the checks don't replace buying direct.
Source: https://trezor.io/support/troubleshooting/device-issues/firmware-is-already-installed