For most transmissions of digital information (even those here on earth) there's a concept of a "checksum". Basically at the end of every message, there's a special number, and you can do some math on the rest of the message to get that same number. If anything happened to change or damage the message in transit, the math doesn't work out and so the checksum fails.

I would assume Voyager works in a similar way so every time it receives a message it will compute the checksum and see whether it matches