First, assess what they do and do not appear to know. And believe. Know is obvious. By believe I mean the sort of person who understands a torque wrench just fine, but doesn't believe they need to use it, and when say a cement anchor pulls out of 8000psi cement, there must be some mystery cause unrelated to their use of an 8 foot cheater bar.
(By the way, I've learned lots from books, and of course lots from Practical Machinist, and lot of learn by doing... I do not mean sound somehow special expert.)
Second, take what Willeo says above and carry it further - people overfilling bearings with too much grease? Why? Is there a chart that says how much to use? Pictures of too little, just right, and too much? That level of thing.
Third, a race mechanic school I knew of, and the robotics clubs I host now, spend a lot of time on things like "if you put a really long wrench on a tiny screw you will just break it" and "you must consult a chart to select the tap drill, the #8 drill is NOT the tap drill for the #8 bolt" and so forth...
Remember that *you* most likely know lots of things those being trained don't know, AND you don't know you know them, nor where you learned them.