I have no intention of dumping anything onto recruiters. At their worst, they earn whatever scorn and ridicule comes their way. At their best, they provide a valuable service. The majority fall somewhere in between.
In my opinion, trying to find a standardized test for the controls field is an exercise in futility. For one reason, the test you would give for a Controls Engineer in a wastewater treatment plant would be different than the test for a Controls Engineer for an OEM of capital machinery. For another, the field is constantly changing. Questions that were important a year ago may be less so today and totally irrelevant a year from now.
In my opinion, the most important thing you, as a recruiter, can do to add more value to the services you render to your clients is to develop enough understanding of the fields for which you are screening candidates to be able to make that informed judgement I mentioned in my earlier post. I applaud your initiative and apologize that my inner curmudgeon got the better of me in my first response.