My MIL just bought a fountain for her cats and they love it.
Anyways, besides the skin tent, you can also feel the gums, as long as they are nice and moist then hydration is good, if they are tacky, then that could mean that the kitty is a little dehydrated.
Wet food is a good source for some extra moisture.

(Cats should drink ~ 1 oz of water per pound of body weight.)