We don’t but everybody’s reporting it was a torpedo. And, I suspect they have some sort of sound recording of high speed screws usually associated with a torpedo. Even if it was a mine...it means the NKs aren’t putting out any kind of warning that “Hey, we’re mining the NLL!”...either way, this gets ugly fast. War? I doubt it, but a replay of the border incidents of 1966-1969 or the Axe Murder Incident? Very possible.
http://en.wikipedia.org/wiki/Second_Korean_War
http://en.wikipedia.org/wiki/Axe_Murder_Incident
The Russians have mines that launch torpedos.