I sometimes guide people at either of the two cave diving sites in the area that require a dive guide. When I am acting in the role of a guide, I usually help plan the dive even though at the end of the day, the clients have responsibility accepting the plan. This includes recommending which gases to bring, the quantities of the gases, dive planning with decompression strategies, etc.
I don't think it's unreasonable to ask for proof of qualifications of a person you are hiring as a guide and I would not feel insulted if a client asked me for such proof.