give the potential tech a log sheet of pressures/temperatures/amp draw/etc. ask him to tell you what is right/wrong with the unit. do not give him everything though...leave some info off and see if he asks for it. if he does ask, give him the correct info and see how he does.
this is not exactly hands on, but i do it frequently over the phone as tech support. if he can do a good job with just a log sheet, he should do pretty well with hands on. then hire him/her as a temporary person for 90 days...if they do not like you, you don't like them, or it isn't quite what everyone thought...no strings attached.
How to make the perfect "Half-Hitch" knot or any other boyscout knot in 3 easy steps...
1. Remove your meter leads from the meter and very carefully return them to their storage case or bag.
2. Wait 2 seconds
3. Very carefully remove your meter leads from their storage place and enjoy your new knot!