As far as I know those optics are the only sensor. They must also be used to detect if the can is present.
Maybe the cycle of blocked, then open, as the can cycles when power is applied and when a bill is inserted is the key.
If the can is full that cycle won't be seen and the bill acceptor will not allow any more bills.
If the can is not present the cycle won't be seen and the bill acceptor will not allow any bills.
Try that, block the optics then unblock them and see if a bill drops through.
Figure out the proper timing for valid detect and re-create with some electronics. You will probably also need to detect the stacker cycle motor is
on before doing the optic dance simulation.