Until decrypted, a TrueCrypt partition/device appears to consist of nothing more than random data (it does not contain any kind of "signature"). Therefore, it should be impossible to prove that a partition or a device is a TrueCrypt volume or that it has been encrypted (provided that the security requirements and precautions listed in the chapter Security Requirements and Precautions are followed).
When formatting a hard disk partition as a TrueCrypt volume (or encrypting a partition in place), the partition table (including the partition type) is never modified (no TrueCrypt "signature" or "ID" is written to the partition table).
There are methods to find files or devices containing random data (such as TrueCrypt volumes). Note, however, that this should not affect plausible deniability in any way. The adversary still should not be able to prove that the partition/device is a TrueCrypt volume or that the file, partition, or device, contains a hidden TrueCrypt volume (provided that you follow the security requirements and precautions listed in the chapter Security Requirements and Precautions and in the subsection Security Requirements and Precautions Pertaining to Hidden Volumes).
Além de inventar um disco rígido invisível, isso é o melhor possível.