I thought this was a great episode and a spectacular start to the last set of episodes. I think the final Cylon is actually Ellen and that she isn't just an older version of Number Six. Why would Number Six age but none of the other Final Five?
I wasn't expecting the 13th Tribe to be Cylons at all. As for how they could tell, we've already seen Baltar come up with some sort of biological test in previous episodes that let him differentiate between human and Cylon tissue. The exact differences haven't been explained. At this point, the definition is getting a little fuzzy but we'll probably have more details in the upcoming episodes.
They did signal Dee's suicide by featuring her in the "Previously on" segment. Just like Cally, the character disappeared for many of the previous episodes. Then when she dies, she plays a major role in that episode. But I still wasn't expecting her to shoot herself when she did. I thought maybe she had turned the corner when she went on the date with Lee.
The episode was threatening to turn into one of the most despairing and bleak episodes in TV history but there was a small glimmer of hope at the end when Adama pulled himself together and started the Colonial fleet on a new quest. If not for that new mission, this truly would have been going where no one (or no TV show) had gone before.