A378 Addendum: It's my belief that Detect Curse was written with only the spell Bestow Curse in mind, unless there are other rules written in the source book it came from. There are plenty of other creatures that bestow curses that don't have CL or spell-level attached to the effect (Mummies, Lycanthropes to name a few). You can use Psyren's suggestion, although I am also having difficulty finding the aforementioned rule.
My suggestion would be to treat Detect Curse as Detect Evil instead, with the aura being based of the HD of the creature that placed the curse, following the same aura strengths as either undead or outsiders. (Whichever you feel is more appropriate).