Nikki,
The paste stuff is called Petro-Malt, and you should be able to find it at most pet stores, although my mom has had to resort to getting it at a feed store in the past, of all places! From my experience, it is quite safe; I remember my parents using it with our cats when I was a kid, and there never seemed to be a problem. The good thing is, cats usually like it, love it in fact!
As for hairball symptoms, if you cat is coughing a lot or you can see the cat's stomach and sides moving in and out (I guess they can't breathe as well because the hairball's stuck in there), those are pretty good signs that your cat MIGHT have a hairball. Another good sign is if the cat is retching a lot. Usually the cat will eventually get rid of the hairball on its own, but treatments do help make it easier and are good for preventative measures.
Hope this helps!
_________________
In loving memory of Lucy, one of the sweetest cats ever.