I have to agree with LilykinsLove; either dogs or cats can speak to each other, or Crookshanks is just brilliant. But at the same time, Sirius said that Crookshanks knew he wasn't a dog... so I'm leaning toward Crookshanks being brilliant.
Also, remember that magical animals tend to be extremely intelligent. Owls, for example, know exactly what their owners are telling them.
Oh! And JKR said in an interview that Crookshanks is part Kneazle. Here's what the
HP Lexicon has to say on them.
Quote:
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This very intelligent cat-like creature can detect unsavory or suspicious persons very well and will react badly to them. However, if a kneazle takes a liking to a witch or wizard, it makes an excellent pet. The kneazle has spotted fur, large ears, and a lion-like tail (FB). Crookshanks is part kneazle
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Probably his Kneazle-ness allows Crookshanks to understand that Sirius is trustworthy and that Peter is not; therefore, he wants to help Sirius.
Yeah. *nods* Hope this helps!
Fenn