The ADV7842 can indicate if the input is encrypted or not.
This can be read back in the HDMI Map
Register 0x05[6] - Content encrypted.
Register 0x04 [4] – HDCP Key Error ( this will be set if for example when using a non- HDCP part ,and trying to establish authentication).
From my experience, It is very common for DVD players to encrypt the link regardless if the content needs protection or not.