(you|one) can never tell

used to say that one can never be sure of something

In the ever-changing world of technology, you can never tell which new innovation will become the next big thing.
When it comes to people's behavior, you can never tell how they will react in a given situation.
Weather forecasts are often accurate, but sometimes, you can never tell when a sudden storm might appear.
In the world of art, one can never tell which piece will become a timeless masterpiece.
When it comes to investments, one can never tell for sure how the market will perform in the long run.