Measuring breathing interoception
One option is to use inspiratory breathing loads. These can be somewhat technical to set up, another option that is simpler would be to use the Power breathe device. An experimenter could pre-manipulate the loads, then hand it to the subject and get quick magnitude estimation ratings, as well as apprehensive expectation ratings beforehand, for calculations of difference scores. For an example of this sort of approach applied to multiple stimuli, see this paper by Edwards-Duric 2020).
The basic idea being that you are measuring respiratory sensation using a contact that is separate from the only individuals respiratory motor control