How can a script check if it's being run as root?

Question

I'm writing a simple bash script, but I need it to check whether it's being run as root or not. I know there's probably a very simple way to do that, but I have no idea how.

Just to be clear:
What's a simple way to write a script foo.sh, so that the command ./foo.sh outputs 0, and the command sudo ./foo.sh outputs 1?

Answer 1

#!/bin/bash
if [[ $EUID -ne 0 ]]; then
   echo "This script must be run as root" 
   exit 1
fi

Answer 2

#!/bin/bash
[[ $(id -u) != 0 ]] 
echo $?

Answer 3

#!/bin/bash
uid=`id -u $USERNAME`
if [ "$uid" == "0" ]
then
    echo 1
else
    echo 0
fi