Fake ones are easy to recognize, majority of the time they try persuading people to follow them for years. Real one will not be known to anyone except in Mecca at the Kaaba when scholars will find him and press him to accept leadership. Also there will be an dispatchment of battalion from 'Sham' region that will be swallowed by earth on their way to Mecca, among other signs.
Salam, I'm not entirely sure on 'fard' but I believe scholars suggest it is and some ahadeeth point to so. He is not savior or messiah. Allah will help him restore Islam in Muslim world right before final major signs. To be honest it is probably Allah proving to mankind how he can change conditions on earth and raise a righteous peoples out of a time where it seems impossible.
Basically once it becomes apparent that the scholars and Ummah agreed on him, and the conditions were met. At that point we have to follow. If we just decide not to for rest of our lives even thought conditions are met then yeah it becomes a problem there.
I don't think Bukhari, but possibly Muslim. There are other hadeeths and scholars have agreed on it's 'tawatur' and agreed it is real and will happen. Some older famous scholars and modern ones, like Ibn Baaz and Albani have suggested so as well.
Is the Mahdi real or not?
The ahaadeeth concerning the appearance of the Mahdi reach the level of Tawaatur in meaning (tawaatur refers to a hadeeth which was narrated by so many people through so many isnaads that it is inconceivable that they could all have agreed on a lie). The imaams and scholars of this ummah have stated this, and a few of their comments are quoted below:
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You should be aware that some liars have fabricated ahaadeeth about the Mahdi, and others have even claimed to be he, or that he belongs to a group other than Ahl al-Sunnah wal-Jamaaah. Some of these "false mahdis" have tried to deceive the slaves of Allaah for some worldly gain and to distort the picture of Islaam. Some have started movements and revolutions and gathered those people and supporters whom they were able to deceive. Then they were destroyed, and their lies and falsehood were exposed. None of this affects the belief of Ahl al-Sunnah wal-Jamaaah in the Mahdi, upon whom be peace, and that he will undoubtedly appear and rule the earth in accordance with Islamic Shareeah. And Allaah knows best.
https://islamqa.info/en/answers/1252/is-the-mahdi-real-or-not