Of course you can check it by checking achievements. Every fully unlocked area gives you one achievement. Anyway in my opinion this way takes too much time. You need to know the name of the achievement, so you have to check via internet and so on.
Much faster is to check it in your Cyclopedia!
Name of areas which is impossible to explore and unlock is in grey.
Name of areas which we didn't start to explore yet is in white.
Name of areas which we started to explore, but didn't finish yet is in purple.
Names of areas fully unlocked is in green.
It is the best way to check which places you still need to unlock and which of them are already unlocked :)
PS. "and navigating this Cyclopedia map is driving me insane" -> I'm not sure if my answer is correct then, hah :D I answered too fast, reading only a headline. Let me know in the comment, I'll hide it if necesarry. Np.