Что-то типа этого:
if (strstr($_SERVER['HTTP_USER_AGENT'], 'Yandex')){ $bot='Yandex';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'Googlebot')){$bot='Google';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'Slurp')){$bot='Hot Bot search';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'WebCrawler')){$bot='WebCrawler search';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'ZyBorg')){$bot='Wisenut search';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'google')){$bot='Google';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'scooter')){$bot='AltaVista';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'StackRambler')){$bot='Rambler';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'aport')){$bot='Aport';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'lycos')){$bot='Lycos';}
else if (strstr($_SERVER['HTTP_USER_AGENT'], 'yahoo')){$bot='Yahoo';}
if($bot !=""){
echo 'MetaTags';
}