With a hex browser:
CR2 is actually an extension of TIFF, so a CR2 file starts with a TIFF header. This means that the first 4 bytes are either 49 49 2A 00 or 4D 4D 00 2A. If you browse the file with a hex editor there are some recognizable things in the first bytes of the file:
If you see completely random junk instead or swathes of zeroes the file is probably FUBAR.
With tools you probably have at home:
- The file should of course have a reasonable size for the camera (around 25-30 MB for my 20Mpx camera).
- A good CR2 file doesn’t compress well. ZIPping a CR2 file yields a ZIP of about the same size (98-99%). If the ZIP file is significantly smaller, then the data in the file isn’t as random as it should be (probably long sequences of zeroes) and likely invalid and not worth recovering.