Now that it's known that Cylons have fiber-optic links in their wrists, how hard can it be to detect them?
well...like someone said, this HAS been discussed to death, but the real answer is that there IS really no answer. Its never been explained in the show. All we know is that the Cylons "LOOK" human to testing equipment, but ther spines glow (sometimes) they are able to communicate with other cylons, and can stick fiber into their arm, and get a signal.
so....either they're somehow able to mask the parts that aren't "human", or you just have to get over yourself and remember this is just a TV show.
personally I don't see how anyone can say that the cylons are "undetectable" and 100% human. this may be based simply on the fact that cylons human HAVEN'T been detected, not that anyone has tried and failed. Remember that until well into season 1 most of the BSG world had no idea that there WERE human cylons, so the statement that they're indistinguishable from humans may be based on the fact that they have managed to hide the special bits well nough to have avoided detection thus far. The only person that's really TRIED to detect them was Baltar...and we never really get a clear indication of 1) whether he was actually able to do it 2) whether he was NOT able to do it, and merely worked that into his psychosis by thinking he's protecting cylons 3) whether this was an easy thing or a difficult thing (he does take a lot of time to develop it, but that could be just because he's crazy, stalling, or whatever)....
so at this point, we really don't know. it may be eventually explained. it may not leaving many dorky people like ourselves to spend many hours trying to explain plot holes in a TV show rather than just saying "hey, its a plot hole...we don't HAVE to justify it for the writers if they can't be bothered to do it themselves. They get paid to do this...we don't."