It's different for everyone. You have to know your body and know the symptoms of dehydration. I drink 20oz of coffee in the morning and then don't drink anything again until late afternoon. I do consume a lot of fresh fruit which is full of water. However, I know some ostomates who always have a bottle of water with them. There is no one size fits all for hydration or salt intake. Again you will need to determine what works best for you.
I don't have any special tests at my annual physical.