Question About 1909 Half Eagles

Hello all,
As some of you may know, I had a debacle a while back with a 1909 Half Eagle that I bought as a Philly strike, but it turned out to have a mintmark after all. The mintmark was quite worn, and I thought that I might have gotten lucky and received a 1909-O, but once I sent it to ANACS, it came back a 1909-D :(. So here's my question. Does anyone out there know of any diagnostics aside from the mintmark that would enable someone to tell the difference between a 1909-O and a 1909-D? I've tried looking at many pics of both coins....and some of those D mintmarks are really scary once they wear a bit....any help that anyone could give me would be greatly appreciated.