I might have mentioned this before, but one of the classes I teach is Respiratory Protection, which includes mask Fit Testing.
For the Fit Testing, each person puts on a mask, and and then I spray a bitter mist solution in front of them. If they can smell or taste bitterness, the mask either does not fit or is worn improperly. The person completes a number of movements while wearing the mask: moving head side-to-side 10 times, up and down 10 times, reading a paragraph out loud, etc. if at any time they smell or taste the bitter they fail the test. We start over with a different mask size, or ensure that they are wearing it properly.
Before COVID Fit Testing was conducted mostly on two types of employees: medical personnel who needed to wear N-95 masks when taking care of TB patients or patients with other diseases they were airborne transmission, and people who worked with chemicals, in paint booths, etc. and wore respirators or masks.
After COVID, we also began training the general public how to wear masks properly. Many businesses wanted to make sure their employees were wearing their masks properly.
I have a case of masks in different sizes, and during the testing we make sure each person is able to find a mask that fits properly. We provide each of them with a card that includes the date the Fit Test was conducted, and the proper mask size Then the employer orders a supply of masks in each size that is required for their employees.