If you do not have people in house that are experienced in electrical and plc then you may want to find an employment agency, especially if they specialize in technical, to assist you.
Another option is to purchase "tests" designed for the specific job. Example:
http://www.totaltesting.com/industrialskillstests.shtml
There are others and a technical employment agency could assist you with this too.
If the above is not an option then you may have to rely heavily on his stated "verifiable" experience. The problem with questions you may ask is that you dont know the answers. If the position is not for a full time programmer but more for electrician with plc skills then worry about the electrical part the most.
Anyway I ask people for electrical and plcs to draw a holding circuit in relay logic and ladder logic for motor control, you would not believe how many can not do this. In ladder it will look something like this:
Stop Start Motor
|----| |--+-----| |-----+------( )---|
| |
| Motor |
+-----| |-----+
This is a basic drawing for a reversing motor using older standards in the US, the proper method now is to have the OL(overloads) on the supply side.:
Another simple version can be seen here:
http://www.patchn.com/electrical.htm
It that picture where L1 and L2 feed the control circuit there should be a transformer or DC power supply similar to the picture above. The transformer is above the Stop button its the 2 flat lines with the half circle lines over and above them. A transformer is shown here:
http://www.patchn.com/symbols.htm
You could ask about symbols, they should be familiar with those:
http://www.patchn.com/electrical.htm