One of the most intriguing books on numismatics I've read in recent years was "Numismatic Forgery", by Charles Larson.
Although it's been a while since I read it, I recall being not so much frightened, but shocked and taken aback at just how far numismatic forgers are prepared to go in order to make the objects of their desire. In the book, Mr Larson "takes the reader into the clandestine workshop of the criminal forger, and explains the methods and techniques that every serious coin collectors should know about before they purchase a rare or expensive coin."
You might be curious as to how he was able to come by this information, well it turns out that our mate Charles was a prison warden, and one of his charges was a gent widely regarded as a "master" forger. He built up a body of knowledge over a period of years, the results of which are published in this book.
If you have any kind of serious interest in rare coins, you should get yourself a copy of this book - no arguments, just do it. I don't get a commission from selling it, and you can get it from either Amazon or the publisher directly.
It's a bit hard to share information in a book on a blog, but a recent post on the NGC website does explain a little about (just!) one of the methods used to counterfeit coins - the spark-erosion method.
Although this method of counterfeiting is somewhat out-dated (and thus likely to appear on any potential purchases), I believe that it is well worth looking over the images on the NGC page just in case you ever happen to come across anything in future. It might not appear on an Australian coin (they are pretty crude), but you never know what your family and friends may come across in the years to come.
The article also describes the process (very interesting in itself I have to say), as well as tips on exactly what to look for.
Well worth visiting!