You can never tell
Definition 1 of 1
Pro Tip 1/3Unpredictable Situations
Use when outcomes cannot be guessed, often due to too many changing factors. 
She might win the competition, but you can never tell.
Pro Tip 2/3No Certainty
Emphasizes that you cannot be 100% sure about the future or someone’s actions. 
He says he will join us, but you can never tell.
Pro Tip 3/3Casual Tone
Appropriate for informal conversations rather than formal or written contexts. 
You can never tell how long the meeting will last.