I handle much of the WWI material for this website ans also work closely with divers to locate and identify submarine wrecks in the English Channel and North Sea.

WWI U-boats are usually identified via propeller markings. How the props are marked depends upon the shipyard that built the boat but they usually simply give the boat's number (along with other details). It's not a serial number thing.

This doesn't work for WWII U-boats. Instead what's done is that the wreck is surveyed (filmed) and key features are compared to known construction details and photographs to determine the boat's ID.

Of course, all this is done fully considering where the wreck is. Unfortunately that's the problem here. There just aren't two U-boats missing off the New England coast. The closest is U 184, east of Newfoundland after November 21, 1942. U 857 (or U 879) could be missing somewhere off the U.S. East Coast but that's a very end of the war loss.

So to get an additional war loss or war losses off new England, our current best understanding is going to have to be wrong. That's possible, it's certainly happened before (U 869), but also probably odds against.

So basically, dive or scan what you think you have. If it turns out to be a submarine, determine it's key features (determine what type it is). Then worry about how to distinguish between specific boats of that type. If it's a WWII U-boat, I can put you in touch with someone that is very qualified to distinguish between the possibilities based upon a film of the wreck. He literally wrote the book on WWII U-boat losses.

Best wishes,
Michael