While it's true in a mathematical sense that one may not be able to prove that a sequence of seemingly random bits is an encrypted files that is not true in a legal sense. The law isn't about proving things 100%. The law is about weighing the available evidence and proving things to various standards (preponderance of the evidence, beyond reasonable doubt, etc).

For example if if there are server logs showing I downloaded illegal files, and there are people who testify that I talked about downloading illegal files, and there is non pre-installed software on my computer that is used for encryption then probably that sequence of random bits is an encrypted file.

Maybe you can't prove it mathematically, but you can prove it legally.