Over at Poliblog, Dr. Taylor has a post entitled "BSGblogging: Who’s a Cylon?" Now, before I start in here, you must remember that all this speculation is subject to change under the Laws of Bad Science Fiction Writing (which the BSG writers have avoided thus far), under which rules of continuity or logic can be thrown out the window by a cheap plot device (e.g. an apparently dead character was actually not killed, but his clone was; whatever happened in season four was simply a hallucination due to hitherto unknown alien mind-control powers; travel to and from alternate universes; etc). I'm sorry to say that SciFi has been known to operate under the Laws of Bad Science Fiction Writing, as in a particular series of episodes of Farscape in which Scorpius was "killed" three episodes in a row. Twice, his survival was explained implausibly ("I managed to outrun that nuclear explosion") -- and, as I recall, the third time they didn't even bother explaining it. So far, though, BSG has kept with logic and continuity, so we'll give them the benefit of the doubt.
I would reject the whole "trying to procreate" basis as a cylon test, since we do not really know who is trying to procreate. The cylons may very well simply take sexual pleasure for its own sake. Actually having procreated is a partial disqualifier, since it disqualifies one of the two participants. In other words, the whole "Adama has procreated and is therefore not a cylon" only proves that either he or his late wife was not a cylon.
Therefore, I agree with Taylor's statements that Helo, Starbuck (the egg harvesting), Bill and Lee Adama are not cylons, but I think we cannot know about the Chief. He had sex with Sharon, but she did not become pregnant.
That being said, we can know for certainty (excepting the Laws of Bad SciFi Writing) that Adama, Lee, and Col. Tighe are not cylons -- because they have known each other for too long. We have seen all three of them aging in flashbacks. Lee was born and grew, and Adama and Tighe share memories decades old. The only way any one of them could be a cylon would be if all three were cylons and shared fake memories.
They keep trying to throw this Baltar red-herring out there, but that makes no sense either. If Baltar were a cylon, Number Six would never have had to seduce him and sneak around in the Defense mainframes -- they would have simply activated him and had him do it himself.
Then, there are the "this is just too good a storyline" non-cylons. As far as I can tell, we don't know enough about Ellen Tighe to know how far back Col. Tighe's memories of her go. Nevertheless, she can't be a cylon, because she's far too destructive a force for the writers to get rid of her by shooting her out of the airlock. No, they're not getting rid of her that easily; she's too deliciously bad.
Taylor writes:"So, really, I am going with Billy for sure, Gaeta maybe, and more likely than not the rest are in position we have not yet seen. I think that one will end up being planted in Zarek’s cabal in some capacity." That sums up my position exactly. Besides, have you ever met anyone named "Billy" who was NOT evil?
In just an aside, the blogsphere seems flooded with academics (and a few medievalists) who are quite taken with BSG. Somewhere long ago I read some medievalist (I forget who) who was blogging about BSG and the Aeneid. I've been since thinking about that, and may post something on that topic in the future.