It can't. There's not much more to say, even with the current anticheat it's no way to be certain a real human being clicked a real mouse. As the other comment

It can't. There's not much more to say, even with the current anticheat it's no way to be certain a real human being clicked a real mouse. As the other comment shows, you can start adding more and more invasive monitoring solutions but this likely won't be tolerated by many and will eventually be bypassed anyway.