Vampires do give themselves in some ways this one being one of the easiest.
They also don't drink giving them the "Haven't take a drink for some time" tag at the preferences screen.
Well, the advantage of this over the drink method (which was mentioned on the wiki) is that you have to check the preferences screen of each dwarf for that method. With the bedroom method you can go to any bedroom and 'a'ssign to get a list of which dwarves do and do not have bedrooms. Having gotten the list, you can then narrow it down further with those other methods.
See, the reason I'm posting this is because the wiki doesn't list this as a method for vampire detection. I wanted to get confirmation whether it works, and if it does I'll add it to the wiki. If it doesn't, that means I misunderstood something about the mechanics of vampires or room claiming, and I'd like to educate myself. Either way, somebody will learn something.