scientists must know where memory lies in our brains, all religious stuff aside, how would they know that certain animals have different memory spans than us, such as goldfish? is it a guess then or is there an established part in the brain of a fish that we in our, more developed brains dont have?

Read the link, it's been found out, sistah.

You don't have to know a goldfish's mind to know it's not as good at remembering things. For one, we're more sophisticated then a little goldfish, and have a lot more going on in the brain so it's bigger, and also they could've conducted tests a long time ago that tested a goldfish's memory.