You’re right we can’t.

What we can do though is make educated guesses about the biological structure required to support the thing we call consciousness I.e. a brain.

A mollusc does not have a brain so it’s reasonable to conclude it is not conscious in the same way as humans.

Now if you’d like to argue that the biological structures contained in a mollusc still are capable of containing a complex consciousness I’d be interested to understand that hypothesis and your evidence.