How am I meant to know whether another human has done something intentionally? I CANNOT ever know for certain surely?
Of course you can sometimes know for certain - for example if a ball is going over a player's head and the player jumps two feet into the air and catches it in both hands, I think you can be sure that was intentional. Or if the ball is going into the goal and a player dives full length and tips it round the post - also intentional. Now admittedly those are extreme example and the vast majority of potential offences will not be so clear-cut but it clearly isn't true that you can never be sure.
In the end though, I think you just need to use your best judgment and come to a decision that you feel in your own mind is justifiable. If I think the player has clearly moved their hand/arm into the ball (or into the path of the ball) when there was no other reasonable explanation for why the hand/arm has moved in such a manner, I'm going to see that as deliberate. Similarly if I believe the player has had plenty of time to see the ball coming and get their hand/arm out of the way but has deliberately chosen not to do so, I'm also giving the offence. Also if a player deliberately prolongs an initially accidental contact in order to direct or control the ball that is again an offence for me.
I think the advice about an unexpected ball needs to be re-emphasized and clarified - for me, it is one of the factors that seems to be the most often ignored. If the ball is struck at a player at speed and from close distance or if it is deflected from a very close distance (including off the player's own body) in such a way that the player did not have time to react and move their hand/arm out of the way, then it's not deliberate handing as far as I'm concerned.
Actually I don't think there's too much that needs to be changed from the current law apart from re-emphasizing the requirement for it to be a deliberate choice by the player to either initiate contact or refuse to move their hand/arm out of the way when they had the time and room to do so - and as mentioned, a reinforcement/clarification of the unexpected ball scenario.
Finally, I think we need to get away from an over-reliance on arm position alone as a determining factor (which is where I think FIFA got it wrong in the World Cup). Who can say what is or isn't a natural arm position? When players are in dynamic movement performing extremely strenuous athletic manoeuvres, the arms can end up in almost any position. For me, arm position is really only a factor if the player is standing still - otherwise there are too many variables. Whatever the starting position is, I feel there has to be a pretty clear hand/arm movement towards the ball as it approaches (or a clear failure to move the hand/arm out of the way when there was the opportunity to do so).