How to verify a decrypted cyphertext

[alexpheno () gmail com]

Hello list,

Does anybody know how a computer verifies that it has succesfully decrypted a 
cyphertext?

In the case of Project RC5, they have to find out a string  which is known to be 
readable and probably use pattern-matching algorithms based on some sort of 
dictionary (my 2 cents). But in the case of a encrypted random string how woud one 
know that the solution has been found.

Thanks.