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

It depends on the situation and the personality of the Manager, You have to thank him first for his effort and support, and he must feel himself manager first, then you can explain that something is wrong (Never Say Wrong), but you can say What if?, I suggest!! ...
or something like that.
Be direct and use facts when you explain his mistakes.
Finally, take care because given the fact that "the Boss" is in the position he/she is I would not jump to conclusions and assume they are wrong, there could be factors I am not aware of.
  Thanks Hany Sewilam AbdelHamid Business Development Manager

Practical completion powerfully

If i will tell him his wrong and he wouldn't hear me i would left him to do the false and when he will reach the dead end i'll give him my idea....that's why you must be openminded....in you hole life must hear to everybody around you...every day is a training day for the whole life...

It may arise due to lack of proper training,technology ,attitude and communication of that manager .After identifying the reason of wrong,proper action may be taken.

Actually, if  I am sure that my manager is totally mistaken by his decision or answer then I should indirectly notify him by having documents showing the correct action or proven the right decision and I will till him that I am abit confusing since I thought that we should use this or make this decision so could you please assist me to understand the reasons beyond yours so I can pay attention in future cases.
I see it's not correct to till your manager sharply that you are mistaken but it will be more polite to mention that indirectly  

Trying to convince him by telling him the right method in doing this task  and i will tell him In the form of a proposal to accept the idea that he is worng and accept my proposal  

Dear ,

It depends about case, if this case simple, I can express my point of view in a political way. While if such case seriously have negative business impact, you should be diplomatic first, calm, express all your point of view in a professional way.... if he insist, you can raise such case for upper managerial level escpically, once the case is critical & have very bad BUSINESS impact(s) & you have relative documents for your case.

Whether you're a senior vice president disagreeing with the chief executive, or a junior associate differing with an immediate supervisor, the key to taking opposing positions from the boss is how you package and deliver your opinions.
Of course, you must have something thoughtful and substantive to say.
But even when you have the right case, if you present it with the wrong attitude, it will fall on deaf ears—or offended ones.
You could end up banished to a career gulag.
Treat the boss like you would a customer

Should be treated with caution

Without any hesitation, explain the issue to him in a very positive way with maximum documental evidences and once he got convinced, please put our suggestion/alternatives with his permission.
A good manager definitely will accept his mistake and try to rectify it.

Explain him/her the situation, supported by facts.