Recently I bought a coin (that I will be returning) that I suspected from the start was fake; it was simply a little "too good to be true" to not set off my collector instincts! I had to do some research before i confirmed it was a fake though; and this is honestly a scarily good fake, the best I've yet seen.
To sum up transfer die fakes are counterfeits made using a genuine specimen of the coin; meaning that the biggest telltale signs are the perfectly replicated contact marks (scratches, etc.) that should not exist on two authentic coins; here are NGC's articles on the subject that another member linked for me, with some visual aids:
and here is my coin, with the identical flaws replicated on another coin that was recently sold on Ebay as "genuine"; the most obvious repeat was the broken sabre, with the depression exactly at the same place on the saddle's edge; as well as scratches on the horse's rump, hind legs, and field (I am sure there are others too):
Apparently they are the most deceptive type of forgery and I can see why; my fake was almost perfect weight wise, the alignment and rim were extremely convincing. It appears that the only reliable way for us non-experts to detect them is to seek out other fakes to compare them to, which makes fairly obscure coins like this good targets (unless someone else knows of other ways?).