A polite way to call someone a fool? ... I don't follow, why would it it be impolite? As quite a large fool myself I hope there is no negative implication to your use of the word fool. This kind of bias will not be tolerated! That is our word!
Jk
honestly don't ... just don't. Telling a person they are a fool, or wrong, will do you no good, nor them. If they are wrong, hearing that they are wrong and thus a fool, will only make them dig in more. Even proving them wrong will be tricky, unless it is something very easy to prove like: is the light on or off, if it gets much more detailed than this, than the realm of human opinion takes over and there is no proof that will disprove a solid opinion; human condition 101
if it is an issue that really matters, and it is important for them to know, pray that God opens their eyes, and do not come at them head on. It wont do you or them any good. Wait until you have a chance and then simply share with them your opinion and reasons, let them take them for what they want, if they want to know more share, if not, let what you said marinate with them.
K
O
fools up smartypants down