Hidden Mickey pins were originally called Cast Lanyard Series pin. The first series was released at WDW in early 2002, and there were many more pins in the series than there are now. They also released new pins monthly, where now an entire Wave A or Wave B will be released at once. Originally, they were even more hard to find than they are now, and the original pins could resell for a lot of money. The original Dalmatians, Pooh Train, and the USA Donald in particular could sell for $40. And so then someone started counterfeiting them. I have no information to back this up, but my gut tells me whoever made the contacts to counterfeit these pins are how the "scrapper" problem came to pass. Before, there were still high end pins being counterfeited, but I think someone got hooked up with China, and the China people realized they had a new revenue stream.
Disneyland got their first lanyard set in 2003. Their sets featured a smaller number of pins. For DL and WDW, the gold used for series 1, 2 and DL's 2.5 is a more yellow gold than what they use now. This can help determine authentic pins for these series.
In 2004, the silver Hidden Mickey started showing up on pins. But they started doing it mid-release, so some pins in Series 3 were available with and without a silver mickey. These pins will still say "Cast Lanyard" on the back, even if it has a silver mickey.
In 2006, they changed the name to Hidden Mickey pins, and the first series were pins that had been previously released as "Cast lanyards." In some cases, they changed the size of the pin, in other cases the pins are identical except for the words on the back.
In 2011, they switched to the 2 waves format, but you would know all the pins at the beginning.
I think the 2013 pins were the first to have the Mickey waffle pattern.
You can find the lists on ODPT.com but there are some errors. For Disneyland:
http://www.disneyparksmerchandise.com/blog/hidden-mickey-pins-disneyland-resort/ The 2003 pins are actually series 2.5, the 2004 pins are actually series 2, and the series 1 pins aren't listed at all. For WDW,
http://www.disneyparksmerchandise.com/blog/hidden-mickey-pins-walt-disney-world-resort/ the Series 1 pins aren't listed and the 2003 pins are Series 2.
Disney has sold some sub-series of the HM series in blind packages, so if you are really worried about authenticity, you can buy them. Otherwise, the closer you can collect them to the date they came out, the more likely they are authentic. My experience is that the parks stock pins with the current and previous series of pins. If you find pins that are older than that, they get sketchy.
It's a shame what has happened with the counterfeiting situation, because it was a fun in-park activity, and a way for new traders to easily find something that a more established trader who may not be visiting when a particular pin came out to have something worth trading. Now there's so little trust, you don't know where a person got a pin, or if they have the skills to figure out what is authentic or not.