if [ -t 1 ]; then
  echo stdout is open to a terminal
fi

$ ltrace [ -t 1 ] | cat
[...]
isatty(1)                                      = 0
[...]

$ strace [ -t 1 ] | cat
[...]
ioctl(1, TCGETS, 0x7fffd9fb3010)        = -1 ENOTTY (Inappropriate ioctl for device)
[...]