How can you tell if there is a hidden room in an area?

[Omega Blue]

Other than by randomly digging around, which seems silly?

[Aerach]

Earth's Eyes, Vision, the ... star thingy... unique, spamming teleports to see if you appear somewhere interesting (my method in dark crypt)

[Hunter]

Also, in most -- but not all -- dungeons, if you fully explore the accessible areas, you'll be able to see the edges of inaccessible areas somewhere. This doesn't work for places like the Sandworm Lair, of course, and there are occasionally hidden rooms far outside of your view, but it's pretty reliable for the average stone dungeon since there's rarely more than one square of wall thickness separating the areas. Just caps-lock and scroll around the map, looking at the areas of darkness on the edge and you'll see the tell-tale signs of a traversable room or corridor illuminated by your light when passing, if one exists.

Also, hidden areas in some of the vaguer zones -- where the accessible areas don't take up the entire map, like Old Forest -- are, afaict, rare or non-existent. I can't recall seeing them in the various cave systems or wooded areas. Open areas like Trollmire are pretty obvious. Just look for the dark patches that you can't reach without digging.