Well I can tell you, with my friends, I am not blindfolding myself and telling them to stick stuff in my mouth. That is a recipe for disaster.

Edit: saliva

I have done this as a psychology experiment and they almost taste the same. There are subtle differences. Also, a pinch of salt on the back of a dry tongue is undetectable until a drop of salive touches it.

Does it matter whether the onion and potato are cooked?