The proper candidate needs to be a good shot, have a tactical mindset, and be highly observant. AND, most importantly, they need to have a strong sense of duty and be willing to put their life on the line; for this reason, former military are often the ideal choice. You don't really need a psychological profile test. However, I feel a better test would be to stage a particular scene, photograph it, and then ask the person what they see. One person in the photograph should have a "tell" that they may be about to go for a weapon, to see if the candidate catches it. I assisted with my church's security while I was living in Virginia. I volunteered because, even though the staff were primarily former military and I'm just a civilian, I knew this was somewhere I could contribute. During this time, I observed someone with their hand positioned as if they were preparing to draw a pistol, and immediately alerted others to keep an eye on them. There are certain "tells" when a person is staging their draw that only a person who is both observant and has that mindset will notice.