Update: For monks who are unfamiliar with the Unix file command, it is a utility that examines the contents of a file and reports the file type. It works on both binary and text files, using magic numbers and/or string tables to make its determination. Here's an example:

% file /usr/bin/perl
/usr/bin/perl: ELF 32-bit LSB executable, Intel 80386, version 1, dyna
+mically linked (uses shared libs), stripped
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I tried running file on a few of my C source files, and got back 'C program text'. So file can figure out when an ASCII file consists of C code.

Looking at the manpage for file (on RedHat), I see:

       If  an argument appears to be an ASCII file, file attempts
       to guess its language.  The language tests look  for  par­
       ticular  strings  (cf names.h) that can appear anywhere in
       the first few blocks of a file.  For example, the  keyword
       .br  indicates  that  the  file  is most likely a troff(1)
       input file, just as the keyword struct indicates a C  pro­
       gram.  These tests are less reliable than the previous two
       groups, so they are performed  last.   The  language  test
       routines  also  test  for  some miscellany (such as tar(1)
       archives) and determine whether an unknown file should  be
       labelled as `ascii text' or `data'.
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Two observations: file guesses on the contents of an ASCII file by examining the first few blocks of the file, and the files you tested this on happen not to contain the strings that file is looking for.

I'm not sure what the best solution is. The file extensions will certainly be useful, as others have suggested.