A slider window assembly for a vehicle includes (i) a frame portion having upper and lower rails, (ii) at least one fixed window panel at least partially defining an opening, and (iii) a movable window panel that is movable along the upper and lower rails between a closed position and an opened position. A sensing system is operable to determine when the movable window panel is in the closed position and when the movable window panel is not in the closed position. The sensing system includes a magneto-responsive sensor connected to the frame portion and a magnetic element connected to the movable window panel. When the movable window panel is in the closed position, the sensor detects the the magnetic element. When the sensing system determines that the movable window panel is not in the closed position, an indicator is activated to alert the vehicle driver that the movable window panel is not closed.