How can I tell if someone I know may have a form of dystonia?

Get formal eval. Dystonia is basically sustained muscle contractions causing unusual postures of the neck, face, or limbs, though even frequent blinking (blepharospasm) or a strained voice (spasmodic dysphonia of the vocal cords) are forms of dystonia. If there are concerns about possible dystonia, try to see a neurologist. Dystonia is somewhat rare, and most doctors outside neurology are unfamiliar with it.
Visit a Neurologist. This is best evaluated by a neurologist as there are different types of dystonia.