Aslan, in the chapter before, tells Frank and Helen "And you and your children and your grandchildren shall be blessed, and some will be Kings of Narnia, and others will be Kings of Archenland which lies yonder over the Southern Mountains."

Sooo... it's not officially founded as a political entity yet, but it already exists, and just like Narnia, the name comes from Aslan Himself.

I've always pictured Narnia and Archenland much like my own country, where the borders are pretty "natural", formed in large part by mountain ranges and bodies of water.

Ah. I don't have the book on me at the moment.

But still, how did she know where it would be? 😛

Yonder over the Southern Mountains? It says in the book that they see the mountains and here and there through a gap, the landscape beyond. The way I picture the lay of the land, it's fairly obvious once you have that piece of information from Aslan. (Or at least obvious when you're able to figure out where South is? :P)

Yes, but "it's an extraordinary thing about girls that they don't know the points of the compass." 😉

And anyway, I know them, but the sun mightn't rise and set the same way in Narnia. The world is flat.

Although did they know they were heading west? I don't remember.

Sun's easy to figure out. The point I'm getting at is, the "points of compass" have to do with sun, originally, the magnetic side of things is a happy accident. Plus, apparently, it works similarly even in Narnia's flat world, because Peter could use his compass in Prince Caspian! (That's an interesting idea for a worldbuilding fic, though: how did Narnians discover the workings of compasses? I'd bet dwarfs were involved.)

And yes, they do know they are going West.

I have read that a number of migratory animals can sense the magnetic poles, it could be that Fledge could as well, even if they couldn't tell from the sun and that they were told to go to the west.

Now I re-read this thread, it occurs to me that it might be easier to figure out it has to do with the sun in my own language, where the word for "East" is the same as for sunrise, and that for "West" the same as for "sunset"...

Oh, that's cool, marmota!

Getting back to the original question:
Unless she was a seer, Polly could not have known where Archenland was going to be in the future unless someone told her. The only person that would have known that is Aslan or perhaps someone like a centaur (that could read the stars?). So if you keep in the canon (i.e., assume that this is not a slip on Lewis' part, but rather look into the Narnian world for answers), perhaps Aslan could have given more detailed instructions to Polly (or Fledge) that were not recorded. (Fledge could have told Polly, and>>) ...As they were flying she could have recognized the geography enough to point Archenland out.