Is it possible to know the pH of a liquid without testing it in some way? No.
Is it possible to know the pH without buying specific fishkeeping products that may be overpriced? Yes, use your friend google. Can you get hold of the required indicators or chemicals?
Litmus paper is going to be the really easy way to see if you're outside the basic fishkeeping range, even if it's not specific enough to target your fish based on the pH. That's got to be cheap as chips, plus anyone still at school would be able to lift you a book of it no worries.
