How to spot a kidney problem in cats?

Im very worried my 3 yr old cat might have a kidney/liver issue, we took her to the vet and she stayed overnight getting iv fluids because she was dehydrated. The vet said when she was pressing her kidneys and around there she would cry out. we have already paid 800$ for that, we might get xrays but that is 1500. I need help , now she is home and eating wet food a tiny bit, i put some drops of water in her mouth and she is on pain medication by the vet. She is still wuite sensitve under her belly... what could it be? :(