If the patient isn't able to answer or is unsure it gets a little complicated based on the risk factors, how soon the image is needed, etc.. Sometimes screening imaging of the orbits (eyes) is done if there is a history of metal working or a CT head/orbits (bullet fragments in brain = no bueno), chest X-ray if you can't get documentation on a pacemaker, and so on.
While I don't know enough about metal detectors to know how sensitive they are, I suspect someone has looked into it because there would be a lot of money to be made/saved if you could, for example, use a detector to confidently rule out metal fragments in the eyes. Bear in mind these fragments are <1 mm, and if you are wrong and the powerful magnet moves it even a couple mm you could blind a person or worse!
Metal detectors don't (typically) detect body jewelery so I don't think they are very useful in a clinical setting since they'd have to detect that sort of thing.
Source: I have a bunch of body jewelery and been through a bunch of metal detectors and never set any off.
Well yes the ones used in security checkpoints are probably only set to detect big pieces of metal like guns and knives. It's a pain if they go off every time someone walks through with an earring or a metal button or zipper.
But people do use metal detectors to reliably find bottle caps, jewelry, coins, etc, under several inches of sand and soil.
If the patient isn't able to answer or is unsure it gets a little complicated based on the risk factors, how soon the image is needed, etc.. Sometimes screening imaging of the orbits (eyes) is done if there is a history of metal working or a CT head/orbits (bullet fragments in brain = no bueno), chest X-ray if you can't get documentation on a pacemaker, and so on.