Wasn't expecting a habital zone, just a place to stick a out of the way station.
X-ray and infrared spectra are telltale signs. Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planet like temperatures (under 1000 K).
There is apparently an article on just this subject one SJGs JTAS site. Good thing I am a member still...
IMTU BDs exist in quantities, both in blank hexes and as additional occupants of occupied hexes, but for the reasons quoted above, are effectively useless - but I don't want to use them, I'm just handwaving an acknowledgement of their astronomical existance. You obviously have other plans.