I am very passionate about making sure that my sister facilities, as well as my own, are ready for third party audits.
When I do second party audits, I am perhaps not as professional as I should be. When the person I have in the cross hairs is bumbling about and prevaricating...I just come right and say "well, that's just BS."
Say what you do, do what you say and be able to prove it.
I tend to nit pick. Especially on documents. Nothing sets my "NC" pen off more than seeing documentation that is filled out at the exact minute on the hour, every hour. When I am walking around on the floor and I see this, I make sure I am near that monitoring station at the next "scheduled" check. Nine times out of ten the check is not done when the time is logged. Then we have to have the immediate training on "WHY" it's OK to miss a check, as long as the check is completed and a deviation form is filled out.
As far as tactics go, I'm a big fan of open and closed ended questions. If I ask "so what do you do if x" and I get a 5 minute rambling, incoherent babble, I know the person has no clue what the policy or procedure is.
That being said, when you hit the floor, I have worked in food facilities for 24 years. I am certainly not going to complain about some flour on the floor, nor am I going to complain about a whole bunch of trough grease on a lift assembly if the line has not shut down for three weeks. I might point out that it might be a good idea to have someone clean off that obvious gob of food grade oil that is near, not over, a product zone, but I know what it's like when sales do not allow you to shut down to clean.