Originally posted by Bravely Bold Sir Robin:

They're generally denoted by either a) A dvergr outpost with no dvergrs but a TON of seekers (follow the staircase down), or by a massive staircase with blue lights on it. So look out for dvergr outposts and blue lights. You may have passed up a few mines thinking they were dvergr towers.

Marble staircase. Abandoned towers with wooden staircases are just ruins.
I can't always see the blue lights either, but that might just be me.