You look for what is common job knowledge. In the case of a cook, he might not even know how many barrels the rig produces. What he will know:
-- How does he get to and from the rig? Copter, boat, point of departure? Complications like weather?
-- What's his schedule? For example, two months on and one month off?
-- How many meals does he prepare a day? Does he cover the entire 24 hours shift or are there two cooks?
-- How do supplies arrive and how frequently? What's the cover plan for bad weather when transport can't make it on schedule?
-- Equipment? Size and capacity of refrigerator/freezer?
-- Workers access to kitchen and food?
-- Life saving station and procedure? Drills? Training courses?
-- Unusual sites and sounds on a rig? How large are the windows, sound of the doors, how many stairs?
-- How is his pay structured? Does he have direct deposit? His time off -- how is that paid for?
I would expect complete knowledge from any cook from an oil rig on the above. These are the things that the cook will deal with on a daily basis. Usual way to trick people into thinking you were doing something is master a portion of the above, steer the conversation into where you can have credible stories about this stuff. For example, complaining about Jake constantly going into the kitchen scrounging food, which he knows he's not supposed to do, and how your character has to allow extra food to cover Jake's forays into the kitchen, during those periods when Jake is on the rig.
Way to trip him up would be casual conversations about the above. For instance, my guess is anyone on an oil rig has to have X number of hours of training in fire suppression each year. Your cook has to know this sort of stuff and able to answer it.
Jim Clark-Dawe