Have a small widget machine made, and have them program it. Then you can see how they program, and how well they can learn new products. I would say making a written test in this industry would not be best since products change yearly. I would have the machine have some things on it that they have not used before. Provide the manual and see if they can learn. Someone who has the ability to learn new ideas and concepts is better in my opinion than someone who thinks they know it all already.