How can you determine if an LM hash you extracted contains a password that is less than 8 characters long?

Due to LM's split-hashing design, length information can be inferred from the right half, so the claim that no information can be derived from the hash is incorrect.

The LM algorithm pads a password to 14 characters and splits it into two 7-character halves, hashing each independently with DES. If the password is 7 characters or fewer, the second half is entirely null bytes, which always produces the fixed DES output 'AAD3B435B51404EE' - so the rightmost portion of the LM hash being this constant value reliably indicates the password is 7 or fewer characters long.

LM hashes do not begin with a fixed prefix like 'AB923D'; the left portion varies based on whatever the first 7 characters of the password are.

The leftmost portion reflects the first 7 characters of the password and changes with different passwords, so it is not a fixed indicator of short password length.

Short passwords do not produce an all-zero right half; the null-padded DES computation outputs the specific non-zero fixed value 'AAD3B435B51404EE', not all zeros.