The severity of Brachycephalic Obstructive Airway Syndrome (BOAS) is assessed by means of walk tests. With the help of walk tests, individuals with the most severe symptoms can be eliminated from breeding.
See more on the FKC's walk test at: https://dogwellnet.com/blogs/entry/88-finnish-walk-test-for-brachycephalic-breeds-ready/
All the information on the Finnish walk test can be found here.
The walk test is meant for short-muzzled (brachycephalic) breeds that have symptoms caused by upper respiratory tract disorders. These breeds include Pug, English Bulldog and French Bulldog. The dog's exercise tolerance and the ability to breathe normally are evaluated in the walk test and the clinical examination included in it. In the walk test, the dog must walk a certain distance in a defined maximum time and recover from the exercise within a defined time frame."