Actually we know what would be a release number +\- 3. New spin always starts from last pocket
Actually, not everywhere. For example, in the US they do not even attempt to release from the last number because the wheel spins the same direction on each spin.
Watching to see where the ball is released isn't very efficient. It's more effective to let the ball make a few revs and then view the number below it at some specific location on the bowl.