What is the test for establishing a union as the exclusive bargaining representative of an employer?

1 In a card check procedure, employees are given cards asking them to designate the union as their bargaining representative. If a majority of the employees sign and return their cards, the union becomes the employees' exclusive bargaining representative. See NLRB v. C & C Packing Co., 405 F.2d 935, 936 (9th Cir.1969). Typically, the cards are distributed and collected by the union without the employer's assistance. They are then "checked" for authenticity by a neutral party. See, e.g., id. at 936; Snow v. NLRB, 308 F.2d 687, 689 (9th Cir.1962).

2 The letter agreement stated in full: