Hmmm,...
Trouble shoot and inspect equipment, and it is an on site supervisory job.
So presumably this guy will not be rebuilding whole engines or hydraulic pumps on the bench with his own two hands? Just trouble shooting in the field. And as it is supervisory position, he should have had some prior experience in this field.
I would try to get some cut away or exploded drawings of the type of equipment you use, and start asking questions. What does this bit do, what would be some of the things you would check for a particular problem.
In another approach, ask about hobbies and outside interests. If he has a home workshop and tinkers around with vehicles or machinery, that would have to be be a plus.
If his fingernails are completely clean, be VERY suspicious.