How do you measure quiescent current?
You measure it like you would active current use. Nothing changes. Quiescent current is the amount of current a circuit, that is doing nothing, takes. You would connect it to a ammeter just the same. For very low quiescent currents, you may need a more precise or quality meter. Some devices or circuits have IQs in the micro and nano amp ranges, which a cheaper meter would not be good enough to register properly.