I think of it like this:
Dogs I've owned have shown amazing memorization and training capabilities. I can say "Ok, go get your leash." and they'll find it (not always in the same place) and bring it to the door and wait for me. Or there are the hundreds of various tricks you see many dogs do (Go get me a soda and a bag of chips) that involve a high level of complexity. I forget the name for it, but dogs are capable of abstract thought to the point where they know which rules apply when the leash is on versus off. Human children don't learn that until they're around four. All of these things tell me dogs are intelligent.
Ok, cats. Everything I've just said about dogs will of course be repudiated by someone saying "Cats are too smart to do what you want!" By that logic, my tetra fish are fucking geniuses, because I feed them and they don't do ANYTHING for me (at least the cats cuddle). You can't take not doing something to prove intelligence, it just means they won't (or can't) do it.
So what can we compare? Let's start with cause and effect. My dogs know when dinner time is. It's the same time every day. My cats think it's food time every time I walk near the kitchen and god help me if I'm handling a can because they'll meow for the next 10 minutes. It doesn't matter to them that I HAVE NEVER FED THEM BECAUSE OF THIS. There is no reason to believe that something will happen if they pester me and plenty of reason to think that I'll throw them in another room.
Cats just look smart because they never fail because they never try (or can't try).