At this time, the NLRB only requires unions to provide the following procedures to safeguard your right to pay no more than a limited agency fee:

the union must inform you that you have the right to be a nonmember;
the union must inform you that nonmembers have the right to object to paying for union activities not germane to the union’s duties as bargaining agent and to obtain a reduction in fees for those activities;
the union must give you sufficient information to enable you to intelligently decide to object;
the union must tell you about its procedures for filing objections; and,
if you object, the union must tell you the percentage of the reduction, the basis for the calculation, and that you have the right to challenge these figures.

Contact them yourself.