How could they tell the bones found were Cylon? Some kind of genetic marker? Baltar's "test" took a long time to process and seemed to be based on blood-so how did they know so fast?
That one's easy to answer. The bones were artifically grown/synthetic. They probably lacked the tell-tale signs of natural growth and decay, which is something not easily tested for on a living specimen.
Which reminds me. A really big hint that everyone is a Cylon comes back from a much earlier episode in which Athena plugs into the computers onboard the Galactica and no one really seemed to think it was odd that she could do that. Let alone that she could do that and not have an easily discovered reason for being able to do so.
If they were that unsurprised and were still unable to test for Cylons, then clearly their own biology has to be able to support that kind of thing, too. And I assure you, if you could plug a male jack into an artery, there *has* to be something unusual about said arteries!