How to tell if homebrew is installed on Mac OS X

I use this to perform update or install:

which -s brew
if [[ $? != 0 ]] ; then
    # Install Homebrew
    ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
else
    brew update
fi

brew help. If brew is there, you get output. If not, you get 'command not found'. If you need to check in a script, you can work out how to redirect output and check $?.