You certainly do not want to make it difficult to take a breath. This reduces your list of devices.
About the only method I can think of is to position a tiny pinwheel at each nostril. It must be friction free. It must have a guard ring around it, so the blades do not contact skin accidentally.
The pinwheel will spin one way when inhaling, and the opposite way when exhaling. I can't think of an easy hookup for taking electronic measurements of its speed. Maybe reflected light?
It won't be accurate, unless everyone's nose is the same size.
My idea seems ludicrous, I admit. It's just brainstorming. Not very workable. Nevertheless such brainstorming can lead to something that is workable.