Detect in C if outputting to a terminal

Use isatty():

$ man isatty
ISATTY(3)                  Linux Programmer's Manual                 ISATTY(3)

NAME
       isatty - does this descriptor refer to a terminal

SYNOPSIS
       #include <unistd.h>

       int isatty(int desc);

DESCRIPTION
       returns  1  if  desc is an open file descriptor connected to a terminal
       and 0 otherwise.

Since stdout is always file descriptor 1, you can do:

if(isatty(1))
    // stdout is a terminal

if (isatty (1))
    fprintf (stdout, "Outputting to a terminal.");
else
    fprintf (stdout, "Not outputting to a terminal.");