I think the necessity is for flight crew members to do some methodical air sampling when they believe cabin air quality is inferior.
This can be done with the help of small portable devices that allow drawing environmental air through tiny sample cannisters in a controlled manner. The cannisters can then be sent to a lab for very specific and comprehensive analysis. If observers can take samples at the time they observe - or expect to observe - air contamination, the true facts should be readily ascertainable from these timely samples.
Given the high level of concern about onboard air quality, a modest stream of resources to pay for this type of sampling should be forthcoming from pilots associations, unions, or other "interested parties", including government agencies.
'Methodical' is what makes commercial flying work so well. Same approach will get to the facts for this.