Measure brain activity.
Most variations of vitally impaired citizens which try to maintain at least a minimum of scientific plausibility either have an active brain or something which replaces the function of the brain. So you should be able to measure brain activity (or at least the activity of whatever replaces the brain).
You can use an Electroencephalograph for that purpose. It is a relatively cheap and compact device which gets attached to the head and measures the electromagnetic activity in it. If you want a higher reliability, make it less visible and don't shy away from intrusive methods, you could alternatively implant the detector into the skull.
You could even give them a more advanced detector which doesn't just check if the person is still
alive un-dead but can detect certain brain wave patterns and thus allow the wearer to intentionally trigger the dead man's switch with their thoughts.
If your vitally impaired people don't have brain activity, then there got to be some magic power which controls them. In that case you might be able to somehow measure that magic power.
Another bio-function you could measure is their digestive system. Vampires drink blood, ghouls and zombies eat human flesh. That means they need a working digestive system. The activity of the digestive system might be measurable. This might be easier to do with those vitally impaired people who prefer solid food, because they will need active intestines in order to digest that food. The chunks of flesh moving through their guts should generate measurable movement. Curiously, I can not think of any source of zombie lore which mentions whether or not zombies defecate. Intuitively, I would say they do, because some sources say that zombies must eat to survive and a 100% effective metabolism is implausible. If your zombies consume flesh out of instinct but don't actually get nutrients from it, then their stomachs would quickly burst while eating. While a zombie walking around with an open belly oozing a sludge of stomach acid and half-digested flesh is certainly very zombie-esque*, I can not think of any zombie lore mentioning that this is a particularly common occurrence. Also, zombies shitting themselves certainly makes them even more gross**.
It might be a bit harder for those who consume their nutrients in liquid form. Maybe they have active kidneys and a working bladder?
*I think I got a halloween costume for this year!
**Not going to incorporate that into my costume, though