In "C" how to test if file is a text file?

You cant'. All files are just a stream of bytes. There is no difference between a "text file" and a "binary file".

The only difference is in your code. How you open the file, and with what options, is what determines how the content of the file is interpreted, be it text or data.

You can't. There is no "attribute" that windows stores which determines file type - just the extension which can indicate what kind of programs can open it successfully.
Some files do contain information at the start which says "this is a ..." but it's not compulsory, and in some cases is text based anyway - like an EXE file which will start with either "MZ" or "PE" - but there is nothing preventing a text file from starting with "PET LIST" and passing the "is it executable?" test!

Sorry, the only way to determine if a file is a text file is to read ti and see if it contains "non-text" values. Even then, if it's a Unicode file in (for example) Persian it will contain values outside the "normal text" range.

Quote:

I need to check if the file selected by the user is a text file

The only way I see is reading the file and check if contain match your criteria.
I fear you have to write your own utility.
First, you need to define what is text for you:
English is mostly lower 128 ASCII, but under 32, one can argue that it is not text expect for CR and LF.
But English also include words from other languages like "déjà vu" which comes from French.
When I use csv files, I replace commas with tabs (ASCII 9), is it text for you ?
If file comes from DOS, text that use codes above 128 are perfectly normal for Europeans.
File can also use UTF encoding.

This may need some changes, but appears to basically work (for my purposes).

//====== FUNCTION - CHECK IF INPUT FILE IS A TEXT FILE ======//
int fnCheckIfTextFile(FILE *pfFile) {
  fseek(pfFile, 0, SEEK_SET);
  if (ftell(pfFile) != 0) {
    printf("fnCheckIfTextFile: There is an BOF problem with this file.\n");
    return 0;                           // FAILURE
  }
  int iByteTot = 0;
  int iAsciiTot = 0;
  int cVal = 0;
  while((cVal = fgetc(_pfFileIn))!=EOF && iByteTot < 10000) {
    iByteTot++;
    iAsciiTot += (cVal > 31 && cVal < 127);
  }
  printf("Total bytes = %d, ascii bytes = %d\n", iByteTot, iAsciiTot);
  int iPercentAscii = ((iAsciiTot*100) / iByteTot);
  if (iPercentAscii < 90) {
    printf("This is not a text file\n");
  }
  return (iPercentAscii >= 90);
}