A PostScript file has a unique header.

If you looked at a PostScript file with a text editor or with the more command, the start of the file might look like:

%!PS-Adobe-2.0
%%Creator: O'Reilly 2.1
%%CreationDate: 12/12/94 14/12/40
%%End Comments
/ld {load def} bind def
/s /stroke ld /f /fill ld /m /moveto ld /l /lineto ld /c /
curveto ld /rgb {255 div 3 1 roll 255 div 3 1 roll 255 div 3
1 roll setrgbcolor} def 126 142 translate
360.0000 508.8000 scale
/picstr 19 string def
152 212 1[152 0 0 -212 0 212]{currentfile picstr
readhexstring pop} image

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