Not sure if any of this has been listed yet, but I've got four vampires locked up right now. All were identified by giving them nicknames, and then viewing deity relationships to check for nicknamed vampires.
DF Wiki has an article on identification, and in my experience it's not very accurate.
1.) Vampires who migrate to your fortress do sometimes have an abnormally massive looking list of former group associations in their historical background.
Status: Mostly true. The vampires do have a longer list, but only one of them has a list that could be termed 'abnormally massive'.
2.) Furthermore, they are likely to have very large lists of relatives, none of whom can be found within the fortress itself (in stark contrast to the parents, siblings or cousins whom most dwarves will share their home with).
Status: False. Two of my four barely have relatives at all, and the two remaining each have a relative in the fortress. One is a cousin, one is a younger sister. Both of the related dwarves, as far as I can tell right now, are normal.
3.) If the suspect is married, but his/her counterpart is not present at the fort, this is a very strong evidence.
Status: True for three. The fourth vampire is unmarried.
4.) They may also have an unusually large number of children.
Status: True for the two vampires that have extensive relative histories. Probably true in general. One vampire has 15 kids, the other has 6. One of them only has two, however.
5.) Their biographies may indicate that they 'have the appearance of somebody who is (x) years old'
Status: False, none of my dwarves' biographies say this. Their ages are not abnormal either: 114, 40, 66, and 63.
6.) As vampires do not eat, sleep, or drink, they will never have recent thoughts about meals, drinks, beds, dining rooms, or chairs.
Status: Absolutely true. This is one of the indicators that I've been using to identify them.
7.) In the case of older vampires, particularly those who have immigrated, in the line which speaks about needing alcohol to get through the working day a comment may be added to the effect that "s/he could really use a drink," "s/he has not had a drink in far, far too long," or "can't even remember the last time s/he had some," indicating that they have been sober of alcohol for a time.
Status: Also absolutely true, however they do not always show this line when they show up. Within a season or two it should be evident, however.
8.) Vampire immigrants frequently have very high social skills, and one or more skills at the "Great" level.
Status: True for 2 of the 4. For one of them, it appears that he was actually the only dwarf in his entire migration wave to have social skills.
9.) Another excellent indicator is that all vampires have white hair.
Status: True for one dwarf, however, it's true for the one that says he's 114, which makes it look plausible.
Most exciting, however, is this fellow:
He appears to have been bitten and survived. Going to be keeping a close eye on him to see what happens.