I suppose that, depending on what facet of what area you collect, there might be a mention of such things in specialist works - e.g. in journals produced by the xxxxx Study Group, or in books in the Society library. So you'd be better joining such a society than just hoping the work will turn up on
Amazon or in your local bookshop.
But quite possibly there won't be any such work. And that's the exciting bit; because you're going to become the expert and find it out for yourself.
Firstly there's two sorts of rare. One sort of rare is - you find a cover cancelled in a tiny post office in a village half way up a mountain mainly inhabited by goats. That will undoubtedly be rare. But it's not the kind of rare that will make you or your friends go Wow.
What really makes a cover rare is not merely that there aren't many around but also that it is of some kind of importance to a postal historian, because it's rarity was created by a blip in the postal system. Let me give you an example. Twice Montenegro invaded the Albanian town of Shkoder. On each occasion they stayed only a few months. So a cover with a Montenegrin stamp cancelled in Shkoder is not merely rare, it is rare and very desirable - provided it's also genuine.
So here's where the fun starts. You have to look at the material you collect from your area and start asking questions. Why is that cover stamped with an x dinar stamp when all the others are have y dinar stamps? Why does this cover have no stamp on it? What is that unreadable note scrawled across it in a different hand? Why does this cover use a revenue stamp instead of a postage stamp.
Then start looking at the covers offered on
ebay or Delcampe - especially those that say RRRR! Can you see any sign of its being unusual? If so, try to find the same feature on covers offered much cheaper by other dealers. If you can't then it may indeed be rare.
The more (relevant) history you know, the better. I recently bought very cheaply the world's grottiest looking cover. It was sent from the Serbian town of Procouple at the beginning of 1918. It looked as if a bomb had hit it. And one probably had;because at that time the town had revolted against the Central Powers and was virtually shelled/burnt flat. But you have to know that.
Just so you don't think this is all about blowing my own trumpet I'll tell you the other side. A while ago I bought a card carrying a handstamp I had never seen before. I snapped it up convinced this was the rarest of Military special cancels, and that my place in philatelic history was guaranteed. When I took the trouble to actually translate the Serbo-Croat it turned out to be a souvenir handstamp for tourists visiting the local caves! This will happen to you too; but less and less as you gain knowledge.
Good luck. And I look forward to the specialist catalogue you will one day write.