Theora is a compressed video stream format. It is often used with the Ogg container format, resulting in a file format called Ogg Theora.
A Theora bitstream begins with the byte
0x80, followed by ASCII "
theora". This signature will appear somewhere in most any kind of file containing Theora video.
A simple Ogg Theora file begins with "
OggS", and has the Theora signature at offset 28. But the signature will appear later in an Ogg Skeleton (video+audio) file that uses Theora.
See also Ogg#Specifications.