Question: I could not find any library that, given an hex value, returns true if the given string is a valid ether private key.

Any hints?

Answer: 
As mentioned by Peter, a private key is a random 256 bit blob. It is a common oversight that there're no restrictions.

It has to be valid for the secp256k1 curve, which means two conditions:

* cannot be zero
* must be less than the order of the curve (called `n` and has a value of `ffffffff ffffffff ffffffff fffffffe baaedce6 af48a03b bfd25e8c d0364141`)

In Javascript you can use [ethereumjs-wallet](https://github.com/ethereumjs/ethereumjs-wallet) to do this verification: `Wallet.fromPrivateKey(<yourkey>)` will throw an exception on an invalid input.

Alternatively you can also use the `privateKeyVerify()` method from the underlying library `secp256k1` or do it manually with the big number library of your choice.
**important**: this answer refers to the public address, not private key, see comments.

Every hex string is a valid Ethereum address.

Currently there are no address checksums implemented, so all the `0x<40 hex digits here>` are valid. We are considering adding basic ICAP in Geth 1.4.1 or Geth 1.5 to allow inserting additional information into an address that would prevent mistyping them, but until then as long as it's 40 hex characters, it's valid.