So, I can't actually spot this one in the rulebook anywhere.
When you burn Iron or Steel and see all your handy little blue lines, are you able to work out what the metal you're detecting actually is? Can you tell the difference between iron/gold/zinc etc?
Definitive answer: No. The only way you can tell what metal is being used via Allomancy at this point
is using Copper to sense which metal is being burned.
I do, however, like your idea of making this a Stunt - it doesn't strike me as a game-breaking ability and will only come in really handy in a few circumstances such as those you named. I say go for it!
I'm working on a.. combination lock/safe of sorts that requires pulling the right bits to open it