From what I've read one of the keys to a very well done forgery is "the absence of white" Basically an essay to a proof to a plate to a stamp and then to a forgery all loose detail along the way. If there are fine detail engraving lines in an original and you are looking at an example that seems more filled in it of course can be over inking but also a hint to look closer. Its great to have examples in hand and a good scanner but while online or even in person hunting I am sure many forgeries are to late seen by dealers and buyers.
Here is an excellent write up on the newfoundland pence issues that both sperrati and Spiro had a field day with.
http://www.easternstamps.com/images...tion_FLd.pdf