How do people who decrypt encrypted messages know they have decrypted to the correct message? Do they just keep decrypting until the message is in English (or any other language), or is there a standard, proven method that says that when a certain algorithm is applied to an encrypted message, the original message is reproduced?
I was just thinking about this today and it struck me that it is quite possible to hide a message inside what seems to be the actual message, which could be intended to throw off anyone who might be trying to decrypt the encrypted message without knowing the encryption technique. So with this new insight, it sort of makes it impossible to decide if the message decoded is the original, or another puzzle. Is this a thing, or am I missing something?
I feel I should narrow this down to a specific instance, so that answers are less broad. In the case of a war, when the one side intercepts an encrypted message/transmission, is it always guaranteed that the decrypted message is the original intended message [before encryption]? Say you have a job of decrypting intercepted messages, and are given an intercepted encrypted message intended for the enemy, how would you even start decrypting it or choosing an algorithm that could do it? Is there like a step by step process that the entire process from A-Z depends on and is proven to work?
Also as one of the answers pointed out, how would you know the enemy is sending encrypted coordinates or a message? Or is the same decrypting technique used universally? In essence, are humans so predictable that any encryption technique developed is following some predefined pattern (answer with pattern or link)? I hope this is enough to set some new answers on the right track.