they don't have symptoms of it
What symptons?
Got this from the wiki:
Firstly, there are the consequences of their age. Vampires tend to be high in multiple (4-5+) social, high in at least one military skill, and "great" or better in at least one domestic skill. The biggest indicator of a vampire in v0.34.11 is that they will almost always have more skills (10-15+ easily) at Novice or better than any of your other dwarves. If your new Great Hunter is also a Novice Milker, Shearer, Farmer, Tanner, Carpenter, Stonecrafter, Furnace Operator, Soap Maker, Fisherdwarf, Fish Cleaner, and Fish Dissector... they're almost certainly a vampire.
They also tend to have very long lists of group associations, on the order of dozens, far more than your normal dwarves. They have abnormally long lists of relations and often many, many children, but none of them are present in the fortress (in stark contrast to the spouses, children and siblings whom most dwarves will share their home with). If they are married to a dwarf that is not present in the fortress, this should be treated as especially strong evidence. Note, however, that lacking relatives within the fortress is not a good indicator of being a vampire.
Their personality can also be scrutinized for abnormalities. Their biographies may indicate that they "have the appearance of somebody who is (x) years old," a very good indicator of a vampire in cases where they have too many children or too many civilization associations to be that young.
As vampires do not eat, sleep, or drink, they will never have recent thoughts about meals, drinks, beds, dining rooms, or chairs, leaving their thoughts especially bare and suspicious. In the case of vampires who have been in the fort for a while, 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." This is an indicator that they need blood. In any case, if alcohol is available, it makes an excellent distinguishing mark.
There are two "normal" ways to be absolutely sure a dwarf is a vampire. The first is to catch them in the act; the dwarf will be clearly marked for the duration of the attack (i.e. Urist McUrist, Vampire on the unit list, in red). A vampire does not mind if the player is currently "watching" or even following it. The second is to have a dwarf witness the event happening. This will permanently uncover their identities, but almost always results in a dead dwarf first. More arcane are indicators based on their physical abilities; vampires with injured guts do not vomit, vampires with injured lungs have no problem "breathing", and submerged vampires will not drown (evoking the concept of an olden witch test for finding vampirism). Technically being undead, animated corpses will not evoke cancellation spam when a vampire sees them. An easy (albeit, cheap) way of screening migrants is to send them through a hallway with a zombie on the other side of fortifications/windows in clear sight. Normal dwarves will run away from the horrible sight of a harmless zombie but vampire dwarves will walk right through.
Feeding is treated as a job by the game, and thus appears in the Job List with the text 'On Break' in cyan. It is possible that the genuine 'On Break' (teal) and the fake 'On Break' (cyan) occupy different positions in the Job List.
Looking at the deities that the dwarf believes in (in the relationships screen) can be quite helpful. As long as only "cursed" vampires immigrate (and not blood drinking ones), one of the deities of a vampire should have a "cursed the dwarf [untrue alias] . . ." Lacking this clause in their deities seems to be a clear sign that you do not have a vampire. This non-bugged way of checking a vampire is linked to the "cheap" bugged way of checking of vampires, which is described in the final paragraph.
