MRI with lodged bullet

Lurking RN here. I got a patient the other day who I learned in report has a bullet lodged in his back from many years ago. Naturally I think “alright no MRI for him”, but later find out he had already gotten one. Quick google search shows most, BUT NOT ALL, bullets are MRI-safe. How do radiologists determine that? Unless I guess they happen to know the EXACT ammo (which I would bet is unlikely)