I take it that the scenario here is that the steward has a grievance that alleges that the supervisor has broken a labor agreement and the supervisor is not aware of this agreement.
In this case, the best thing to do would be to try to get as much information as possible and some time in which to review it. What the supervisor should do is request that the steward put in writing what it is that the supervisor allegedly did. The supervisor should then give the steward a date by which he/she will respond to the complaint. The supervisor should then review the labor agreement in question to ascertain whether a violation was committed.
There are two things that should be said here. First, a supervisor ought to be aware of all labor agreements at his or her place of work. Second, if there is a set procedure for dealing with this sort of grievance in place at the workplace, the supervisor should follow that procedure rather than the one that I have laid out.