Check whether your Ledger Nano S is genuine
support.ledgerwallet.com
support.ledgerwallet.com
Eventually someone found out that it wasn't, in fact, tamper proof and reported it. [1]
[1]: I would usually cite these, but since I'm in bed they're all in the post by the kid who cracked it.
Also, I should have used a better title. I'm sorry, the lack of sleep and feeling of being misled probably triggered me into posting this immediately without much though.
If it's not genuine, your Ledger hasn't been tampered with.
If it is pre-configured, your Ledger hasn't been tampered with either - it's been pre-configured. You can reset it and start over with a brand new seed. But if you use their pre-configured seed, the attacker will effectively have the keys to your Ledger.
Neither of these are physically tampering with the Ledger itself and it's impossible to avoid either attack with any device of this nature. You need to receive a device (which may or may not be genuine) and you need to configure that device (which may or may not be pre-configured by an attacker, hoping you just use the pre-configured seed).
However, checking that it is not preconfigured is very important.
[1]: https://cdn.shopify.com/s/files/1/2126/7913/files/Ledger_Tam...
[2]: https://saleemrashid.com/2018/03/20/breaking-ledger-security...