a kneazle was in one of Hagrid's magical creatures class in Book #4. They are used to find stuff, they love to sniff out very shiny things. By stating that crookshanks and Mrs. Norris are half kneazle they are saying that they can use there nose to find stuff like for Mrs. Norris she finds kids doing something wrong.

But i'm not sure that's right, really that is just a cats way. It seems as if they know everything.