Actually No. There is absolutely no way to *prove* that there are two OSes installed with programs like TrueCrypt's decoy operating system. All that you would see is that there are two partitions, and one of them contains the OS to which you just booted. There is no modern experiment (AFAIK) which would help you figure out the presence of another OS once you have booted into the PC.
It's pretty simple to achieve actually. You create two partitions, each of which contains an operating system. You keep the salt in the header of each volume. When the user provides you with the password, the bootloader simply tries to decrypt 4 bytes of data at an offset in the header which should contain and encrypted word 'TRUE' . If it is able to decrypt the word in either of the partitions, it boots into it. So your booting into the OS is determined solely by the password you provide.
Since this is default behavior of the program, there is no need to detect this or anything.
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Actually No. There is absolutely no way to *prove* that there are two OSes installed with programs like TrueCrypt's decoy operating system. All that you would see is that there are two partitions, and one of them contains the OS to which you just booted. There is no modern experiment (AFAIK) which would help you figure out the presence of another OS once you have booted into the PC. It's pretty simple to achieve actually. You create two partitions, each of which contains an operating system. You keep the salt in the header of each volume. When the user provides you with the password, the bootloader simply tries to decrypt 4 bytes of data at an offset in the header which should contain and encrypted word 'TRUE' . If it is able to decrypt the word in either of the partitions, it boots into it. So your booting into the OS is determined solely by the password you provide. Since this is default behavior of the program, there is no need to detect this or anything.