Regarding whether something needs to be considered chametz or not, there is a general rule of ראוי לאכילת כלב - i.e. - if a dog wouldn't eat it, then it's not considered food and therefore it is not chametz.
How old is this standard? I'm assuming that dog's diets have changed in many ways over time. Moreso, different breeds of dogs eat different foods. I assume that therefore this halachic definition has changed, as well. So, if we have something doubtful, how do we know what the standard is now for this halacha? Do we just find someone's pet dog and give to the dog to see if it will be eaten? (My neighbor's dog only eats hot dogs (go figure!) Obviously, that doesn't mean that rye bread is not chametz because the dog won't eat it.)