Data about the hidden volume is encrypted and kept in the second 512 bytes of the volume, where as data about the normal volume is in the first 512 bytes. If th

Data about the hidden volume is encrypted and kept in the second 512 bytes of the volume, where as data about the normal volume is in the first 512 bytes. If there is no hidden volume, the second 512 bytes are purely random data. It's impossible to tell an encrypted volume header apart from random data. It's very much "try, and if you fail, you either have the wrong key or the volume doesn't exist".