How can I determine whether a machine's byte order is big-endian or little-endian?

The usual techniques are to use a pointer:

 int x = 1;
 if(*(char *)&x == 1)
  printf("little-endian\n");
 else printf("big-endian\n");

or a union:

 union {
  int i;
  char c[sizeof(int)];
 } x;
 x.i = 1;
 if(x.c[0] == 1)
  printf("little-endian\n");
 else printf("big-endian\n");