To determine whether or not a responding party has made a reasonable inquiry, you must determine where the responding party searched (what efforts were made), who did they talk to (did they make an inquiry to their legal department, human resources, customer relations, the employees in the chain of command, etc.), and what were the questions they asked.
More often then not I see responses to document requests being done (1) by the one with the highest bar number on the pleading (a.k.a. the newbie associate) and/or (2) by the attorney dictating at their desk instead of taking the time to sit down with the client, determining whom they should be talking to and knowing what questions to ask.
It is my opinion that the person who should be talking to the client and collecting the documents is the experienced senior attorney who has a relationship with the client and knows what questions to ask. If the senior attorney still chooses to delegate, then they need to be “hands on” and take responsibility whether or not a “diligent search” and “reasonable inquiry” were in fact made prior to the response and production being served.