My question is: is it possible to guess the private keys of the BTC paper wallet? is there any clue in the public key to guess the private key?
It is not possible to guess the private key from the generated public key, private key is made up of characters that are long and not guessable.
But, if you mean 'is it possible to calculate the private key from public key' because public is generated from the private key. Still not possible because it is an asymmetric calculation in which what is generated from multiplying private key with a constant point called generated point to give public key is using one way function elliptic curve multiplication, dividing the public key with the constant point can not result to private key generation, it is not possible as it is one way-function.
Also, if you mean if the private key can be brute-forced from the public key, it is also not possible with the modern days computer. The private key space size is a very large number (2^256) which is approximately 10^77, while the visible universe is estimated to contain 10^80 atoms, so brute-forcing a private key just still remain impossible.
It is better you use bitaddress.org to generate the paper wallet as suggested by Quickseller or using using wallets like electrum offline to generate it as pooya87 implied. During generating it from recommended means, also still make sure you generate it safely in an offline environment.