Have her see the ball at release point and follow it until she hits it. Sometimes I have batters swing and stop the ball at point of contact, with no intention of following through for those particular swings. That will tell you if she is seeing it or if it's something else. Could be a mechanical problem, for example.
Seeing the ball is about release point and pitch recognition, which entails identifying the release point in the on deck circle and having mechanics that have both eyes on the ball from the beginning, and having a swing such that your head moves as little as possible. All of that happens in a fraction of a second and the brain fires a swing to try to match it.
At some point, and for major league or college softball pitching it's something like halfway to the plate, the die is cast and the batter can no longer make corrections.
Therefore seeing the ball hit the bat is a myth for all but the youngest of hitters. Stopping the bat won't tell you anything more than if they had taken a full swing.