If you knew a manager is 100 percent wrong about something, how would you handle it?

Thanks 

In this situation you can use ( what if) manner, ask him-her what if this case treat by other manner ( and give him-her the right manner)  

Communication is the strongest part of all. You should convince him/her politely  that the things can be done with another way. For that surely you must have a solid plan. 

I have to agree with Mr. Omar that another point of veiw can be lifted with the manager. If the manager is also the owner of the business, you have to be carefull about raising your veiws and rather insist that your plan or opinion is based solely on increasing the owners business. If your manager and you work for a bigger company, the same can apply unless the managers decision is harmful and purely negligent  and will have a negative impact on the overall company. Then the matter should be brought to the attention of superiors. You cannot just shrug and say "It's the manager's dicision, he is going to have to answer for it' , as this may have a much bigger effect and may in the end effect or involve you for not reporting it.

Thanks for invitation, 

Agree with the answer of Mr. Omar Saad.

Till you get some chance to disclose keep mum , but you have to speak out at once ,

i would provide more information for him , and i would ask for a meeting to clarify my findings

Quite frankly tell my opinion and leave the decision to him

there is always a good way to say no, i will find something according to the situation and tell my manager that he is wrong, in a way that he is learning something new. correcting someone is better than doing the wrong work.