Time Will Tell Meaning

(idiomatic) The results of an action cannot be known beforehand.

Example:   This may not be a good idea, but time will tell.
2011 January 11, Jonathan Stevenson, “West Ham 2 - 1 Birmingham”[1], BBC:
  That was harsh on the former Manchester United keeper after a performance which kept his team in the tie at times. But Grant, so often starved of luck himself, will feel both he and his side were worthy of the good fortune. Only time will tell whether it will be enough.