How can you tell what's missing from your cat's diet??

One of my cats feels this need to eat EVERYTHING. I feed Natural Balance cat food as well as salmon, tuna, white fish, beef, chicken and liver, kidney and heart. She loves it all and will walk in circles meowing at her dish if none of it is available.

But she still goes after everything else. I can't leave food on the stove or counter (like with dinner when I make plates, we sit down to eat but there's still some in the pan). She will jump on the stove and eat it out of the pan. Or this morning my daughter was getting a bowl of cereal. She poured the cereal in the bowl and then went and got the milk out of the fridge. And by the time she turned around, her cat was in the bowl eating the dry cereal!!!

I'm kind of getting tired of this but I don't know what else to offer the dang cat. She's not over weight by any means (and not under weight either--she's a very good, healthy cat). She's about a year and a half old now and is an inside/outside cat.

This isn't anything new either--this has been going on for a long time so I know it's not a case of worms making her hungry and wanting to eat everything.

Any ideas?