Is there a way to tell if a Cards Against Humanity card exists already?
You can use these URLs to see the current version of the cards directly from CAH:


http://cardsagainsthumanity.com/wcards.txt (White Cards)
http://cardsagainsthumanity.com/bcards.txt (Black Cards)


These text files are updated frequently, whenever the base CAH set is updated. 

For some expansions, .txt files also exist:


http://www.cardsagainsthumanity.com/bcards1.txt (Expansion 1, Black Cards)
http://www.cardsagainsthumanity.com/bcards2.txt (Expansion 2, Black Cards)


I wasn't able to turn up the white cards or other expansions through URL hacking.

There are also a couple of 3rd party sites and Github repos which have collected lists of the cards, both the main set and various expansions:


http://mdsc.info/dropbox/cah/
https://github.com/nodanaonlyzuul/against-humanity/blob/master/source/cards.json
https://github.com/samurailink3/hangouts-against-humanity/wiki/Cards