I hope the writers can keep the show reasonably consistent, and not resort to "the final five are Cylons but not like the other 7 so the rules don't apply".
* spine glows red during sex (although Steve remarked that essentially this could just be a casemod for Six)
* Baltar's Cylon detector goes all green
* the (unshielded?) radiation in the nebula slowly kills them
* some kinda plastic fibres?
* can plug a fibre optic cable directly into their arm (!)
* can control the Cylon ship and get data through the Pool of Knowledge (it wasn't clear whether this would work for humans). Number Eight said something about "reading data from contact points" when Galacticans went onto a Basestar.
* the disease from the satellite kills them
* the raiders and centurions have some programming to detect Cylon models
* they can read machine code directly, just by looking at pages of printouts
* there is something "different" about their blood, at least Hera's hybrid blood
So if the writers are going to be consistent, the Final Five should at least:
* not have been exposed for long periods unshielded to the nebula radiation
(which is a bit of a dodgy proposition for the Chief - wasn't he on the station almost continuously?)
* not have had ANY possible exposure to the satellite disease
Otherwise if it's just "some of these Cylons are not like the others" then that's pretty weak SF. How can you have a mystery when the writer keeps changing the clues?
The above would seem to exclude Helo, since he went and shot up a room full of Cylons dying from satellite disease.