Knowledge and illusion

How can people tell what they know from what they think they know.
I like to think I know my self, but thats always challenged by new experiences. When I go abroad people ask me whats it like living in the UK I give them a sure answer, but I'm constantly meeting new people and finding new places in the city I live in.
There is so much that I think I know but I only know of here and now.
Maybe we need learnt illusions to get by, perhaps naivety actually helps us to survive. If I constantly questioned every moment I would not get any where, just because somthing looks smells feels and tastes like a banana it does not mean it is one.