There are only 2 ways to fake a 1909 S VDB, that is to add an S to the 1909 VDB, or to add the VDB to a 1909 S (which would rarely be done, since the 1909 s is rare in its own right. For you, the question is, "Is the S added, and why would someone add an S, and then lose it so you could find it detecting?" I would guess it is likely real. The other scenarios are not likely. The question now is, how do you get rid of as much junk on the coin as possible without harsh cleaning, and how do you stabilize the surface so no new corrosion comes up. I would start with a warm soapy water bath. I would not scrub it at all, but soak it, and then use a thorn from a rose plant to flick off the dirt (rose thorns are quite good for this, they are not strong enough to hurt the coin, but can dig a bit into sand or dirt on the surface). Be careful, take your time, and everything should come out OK...
