read User Agent
if( preg_match('/robot|spider|crawler|curl|^$/i', $_SERVER['HTTP_USER_AGENT'])) )
{
echo 'Is bot or spider or crawler or curl or not human';
}
else
{
echo 'Is human';
}
You can find a list of about 300 common user-agents given by bots here: http://www.robotstxt.org/db.html