What are "ha'Osiyos l'Achor"?
Rashi: The wonders that will come at the end.
Radak: Ha'osiyos are future events. Also "Asah Voker" (21:12, means that it will come). The Targum of Ba is Asa. L'Achor is at the end; so Yonason translates. Initially it says "ha'Ba'os" (22) in the near future. Here it discusses at the end of days.
Malbim (22-23): This is the lowest level of prediction - something that already happened, but its reason is not known, and he tells the reason. Sometimes scientists can do so! L'Achor is from the result to the cause, e.g. if there is smoke, there was a fire. You cannot tell even this - all the more so you cannot tell the future, the way Hashem does via his Nevi'im!
How will we know that you are Elokim?
Radak: "If you will benefit or harm", like those who serve you think that you do.
What is the meaning of "v'Nishta'ah [v'Nir'eh]"?
Rashi: We will tell their words. The Targum of "va'Ysaper" is v'Ishta'i.
Radak: We will tell about you and see you together, like those who serve you, after you will say all this.
Malbim: We will know that you are Elokim if you will benefit or harm, only if we will see it. Kohanim of the Ba'al used to falsely say that it benefits and harms.
