The bug has nothing to do with refugee camps. It's a matter of internal bookkeeping that gets screwed up (and Toady agreed with that in a FotF a number of years ago). Much of time a civ ought to be wiped out its df.global.world.entity_populations entry keeps fluctuating for 100 years or so, after which it freezes on a non zero number. If that number is set to zero prior to the finalization of the world (reaching the end date, typically), while the other conditions for a dead civ are met (no sites [including refugee camps/caves], no living hist figs, etc.), the civ typically ends up dead (that's basically what slabciv does).
It can be noted that for some reason DF suppresses some civs it treats as "struggling" from being included in the list of civs you can embark as. I don't know why, but at least one test showed that such a civ, when added to the list, played as a "struggling" one.
The only known, so far believed certain, sign of a civ being dead according to DF's opinion is to embark and look at the civ screen before any contact is made with anyone else (definitely caravans and goblin invasions, and probably necro ones as well, but visitors probably don't matter, although I haven't looked for that case) is that the civ subscreen is completely empty. If your civ is "struggling" according to DF's reckoning your civ will show up there from the start, and it's added as soon as anyone else is added to it if the civ was dead.
The signs of a civ being dead (according to DF) are:
- Two initial migrant waves of void dorfs for the first fortress (pop caps can block those, if desired), and then never any more migrants (but you can accept petitions from visitors nowadays).
- Never any dwarven caravan (they can go missing for long stretches of time both for "struggling" and healthy civs, so not seeing them for any number of initial years is still no proof of complete absence). The trade liaison is considered "caravan" for this purpose, which means you can never become a barony.
- No army movement, so raids don't work (but there are other reasons for raid parties to get AWOL, so a small number of failures is no proof).
- No monarch. Your civ, being dead, doesn't have any monarch (who'd have to be a hist fig, which would make your civ not actually being dead). "Struggling" civs often lack monarchs on embark as well, but you'll get one either at another site, through a "polite discussion" at your site, or through a timeout at the end of the first or second year (don't remember which one), with a slightly different message. In the last case it seems there's a good chance the expedition leader gets the crown, allowing you for a chance to influence who it will be. A dead civ doesn't get any monarch at any time.
There have been speculations about the possibility to revive a civ, but I've never heard of any report of a success. Any player attempts to increase the civ pop numbers are complicated by the two migrant waves being drawn from existing hist figs, which means depopulating previous fortresses (although the expedition leader/mayor doesn't seem to move away).