I understand that creating a wallet in a secure and offline device, and safely encrypting the private key before keeping backup are the best choices. Will anybody propose me a methodology or an application to guarantee this?
you don't need any methodology or an application to guarantee this, it is very obvious that offline storage are more secure than online storage cz it is not very open to online world's scams secondly encrypting it saves it from other attacks.
according me there is no methodology required to prove it. If you can find any method or app that checks you offline wallet security could definitely be a scam, sorry if I'm misunderstanding your question!