I just finished Battlestar Galactica Season 1. There is a method of identifying Cylons that Gaius uses in the show*, but there are other methods that a reasonable person in the setting would be able to use.
First, we can assume that anyone really famous before the evacuation can't be a Cylon. This mainly applies to Gaius, and possibly Edward James Olmos, (who also probably isn't a Cylon just because he looks so distinctive**) it also applies to those newscasters from the episode where Gaius becomes Vice President, but I doubt they'll come back.
The first episode introduces a method of identifying Cylons: the radiation field that the fleet hides in causes the Cylons to become sickly and relatively weak. Then the show forgets about that method immediately.
Cylons are stronger, faster, and tougher than regular humans. It is impossible to have these traits without expressing them physically. Their bones would be harder to damage, if you punched them, they would suffer less damage, etc.
It hasn't actually been proven that they do it, but if Cylons do indeed upload their consciousness when they die, then they must have a mechanism to generate that signal. That mechanism could be relatively small, but would, by definition, be detectable by standard magnetic-style scanning.
Side note, if a Cylon was instantly and utterly destroyed by a bomb, like in the finale of season one, there'd be no body to generate a signal to upload any information.
Yes, I am aware that this is like a guide to detecting the luminiferous Ether published in 1880. Everything I'm writing is hopelessly out of date. It would be interesting to see what others thought at the end of the first season, but it's not worth the trouble of finding a time machine.
I'm getting a feeling that BSG is going to end up like LOST, where they don't actually have an excuse for some of the "mysteries" they've been generating, and we're gonna be annoyed at all the red herrings.
I'm curious what religious people feel at this show, since the characters can be deeply faithful, but they are faithful to the wrong religion. I guess my real question is, how do religious people feel about works that talk about "faith" in a generic sense, as though all religions were roughly equivalent? The point of religion is not to have "faith" regardless of what your faith is about, it's to have faith in a VERY specific doctrine. Do people feel like their religion is affirmed or mocked by a show that celebrates a different faith?
*There's no way Gaius would be personally doing all the Cylon tests, especially since he became VP. That guy at the helm was his science partner briefly, and he would have technicians assigned to do the grunt work of the test over and over. Of course, that can't be allowed because then the test would actually detect Cylons. (There had better be an explanation for why he didn't let people know Sharon is a Cylon).
**Speaking of looking distinctive, why in the world would someone make a spy-bot out of a lady that is six feet tall, and looks like an anorexic actress? Maybe not all those models dress like prostitutes, but they still look incredibly distinctive.
Also, why do all the Cylons of the same model have the exact same haircut?