Joliet is a Microsoft extension and profile of ISO 9660 (CD-ROM) format to support Unicode filenames. It is often compared to Rock Ridge and related extensions, though it works in an entirely different way.
Robust detection of Joliet requires iterating through all the volume descriptors, but in most cases, one of the following byte sequences will appear at offset 34904 or 36952:
25 2f 40(escape sequence for UCS-2 Level 1)
25 2f 43(UCS-2 Level 2)
25 2f 45(UCS-2 Level 3)