Welcome William, this forum is indeed a fount of knowledge if you can wake the right people. I'm not one of them with regard to decimal, but a search of these pages with respect to a particular coin will often give you the information you require. Unfortunately there is no easy answer regarding modern copies, other than experience and research. There are usually subtle differences in style that an experienced eye can spot, and weight and alloy differences that don't help much if eBay buying. If you are really unsure, buy from a dealer with experience of the issue. But on the positive side, even on Ebay most coins are not fakes.
Good luck,
Jerry