How to find out if a string is random in C/C++

I am working on some Encryption witch create a encrypted string like t7AZChkiBA or ?t7AZDxknCE34F14OuwuXiIcGBIKqSGya03CY9cf9iUSPuCb7owPUzmfTxoBsDzE533S82dvKqm7KmOkREtknHH30z6rLHAHg29COKjX9A6uZxh4fAlrRy

The length is not fixed. How can I find if this string is random and does not mean something?
I don't have a dictionary. I am trying to find some statistical correlation, but I don't know how to do it.