What is the question "Mi Higid me'Rosh [v'Neda'ah]"?
Rashi: Which Navi of the Ba'al told, like me, something that will happen in the future?
Radak: Which idol, or one of its worshippers, told a future event before it happened? If one of them did so, we would know that they are true.
Malbim: Who among you said so from the beginning, and we will know that it will be so in the future?
What is the meaning of "umi'Lefanim v'Nomar Tzadik"?
Rashi: Who said [a Nevu'ah] in advance, and when it comes, we will say that his prophecy is correct?
Radak: Who said [a future event] beforehand, and we will say that he is a Tzadik, and his word is faithful.
Malbim: Mi'Lefanim speaks in the present about the past; me'Rosh absolutely discusses the past. I.e. the Navi said that so will be, and everyone knew. Magicians, what they say is not faithful or clear, they speak, but we do not know that it will be so, only after it comes. Then we say that he said it, and it was correct. However, who can say now that this was said in the past (and it will be fulfilled)?!
Why does it say "Af Ein Magid"?
Rashi: No one among you tells the future, and it is fulfilled.
Radak: We know that there is no [idol, or idol worshipper] who tells [the future].
Why does it say "Af Ein Shome'a Imreichem"?
Rashi: There is no one who will testify when the future event comes 'Ploni, the Navi of the Ba'al, predicted this beforehand.'
Radak: No idol worshipper heard [Kli Paz - even Stam] words from it [and surely not Nevu'ah], for "they have a mouth, and do not speak" (Tehilim 115:5).
Malbim: Even if one of them (magicians) did announce his predictions, no one listens, for they are established to be false.