I could be mistaken, but as far as our enemies go, I think Russia is the only nation who can launch nukes from subs. I dont think China has those kinds of subs, or at least ive never heard of it. I suppose i could google it, but im lazy.
We would know who did it though. Again, they will look at the radiation and have a good idea from that information alone, but the shape of crater left behind will also tell us who did it. Again, its like a finger print. The way we build bombs is different from Russia. All nations build bombs differently, and each type of bomb leaves clues behind that tells us who made them. Admittedly bomb "finger printing" harder to do in cities because, we havent run tons of tests on cities, but we have done MANY tests in remote locations, so we now have a troubleshooting booklet that we can look at whenever an unknown bomb goes off, if that makes sense..