If you are talking about an external hard drive containing a TrueCrypt Volume, then as cited in Moab's answer this shouldn't be distinguishable from a drive containing an un-formatted partition. Forensics would basically only know "it looks like random, so there is a real-world chance of XX percent that it is encrypted" ;-). I'd personally assume that XX>80.
However, if you want a full computer system to be bootable and encrypted, there is no way this could be completely untracable, since it needs to contain the computer code doing the decryption algorithm somewhere in unencrypted form; so if the attacker disassembles the boot loader and whatever code it calls, he/she will inevitably find out if any encryption is being used!
An exception would be if you had your decoy system installed with a normal boot loader and only access your TrueCrypt system using the Rescue Disk which contains the TrueCrypt Boot Loader. Of course you must not let the attacker find out that you own a TrueCrypt rescue disk, so you only moved your problem to a smaller, possibly more easily hidable storage device. If you want to go one step further, you re-download TrueCrypt every time you need it, create a Rescue Disk for booting and then erase it and all its traces afterwards ;-).