"The proof is in the pudding."
That's a distortion of the original saying, which stated:
"The proof of the pudding is in the eating."
It means that you can't really tell something's quality until you have direct experience with it.
So many times, we feel tempted to "guess" or "assume" what someone else is thinking or feeling based on their body language or their facial expression.
Until we have interacted with that person and confirmed the internal experience that corresponds a particular external expression, we just don't know.