1. Words associated with laughter, like hahaha, hehehe, kekek etc
2. Web acronyms such as LMAO, LOL ROFL
3. Funny and synonyms of funny
4. And happy emoticons like :-) ;-) xP
Apart from these there are classifiers trained to identify the most stressed part thereby quantifying the comedy. For example, looooll, lol!!!!! has more points that just lol and so on.
So, the next time you watch any funny video on YouTube make sure that you add some extra letters to see that the video is rated top by Google. You can also help by participating in YouTube Slam to vote for your favorite comedy video.
More importantly is Google reading our comments, too?