The cases that I have seen where there is a doubtful previous diagnosis, or where that applicant states that they have had no symptoms nor medication use for a number of years, are usually sorted out through the use of a respiratory challenge test. These are a way to determine the bronchial reactivity of the individual. There are a number of protocols used, and these vary by regulatory authority (civ/mil/etc). All have there limitations of sensitivity vs specificity, but are the best we have in an objective assessment. I always do these in conjunction with a urinary salbutamol/terbutaline assay to help to ensure that the test results take into account any current medication use.
Cheers,
BM