I have been studying the early issues of Japan off and on for several years now, with the assistance of resources (both print and human) from the ISJP. I am still far from good at being able to capably detect forgeries, and wish I had more time to be able to put into it. (I hope to sometime in the near future.)
So, that said, it is really hard to be able to rule in an early Japanese stamp as genuine, but learning two techniques in particular can help rule out a lot of them (in other words, to be able to determine that they are forgeries).
(1) It was mentioned above, but learning how to spot the little Japanese characters that indicate a forgery (the so-called "signed forgeries") will help you identify a good chunk of the forgeries out there (though by no means most or all of them).
The ISJP has a lot of information about them on their website here:
http://www.isjp.org/ForgeriesOnce you read through that page, then click on a stamp type on the left to see specific examples of where those characters can be found. It takes some practice, but once you learn how to break this code, it really helps weed out the forgeries.
However, this technique only works with those stamps that were "signed". There's still a bunch that don't have those tell-tale signs, and it takes other, more difficult (in my opinion) techniques to identify those. And the techniques vary according to the stamp, so it's a laborious process, and requires a lot of knowledge to really do accurately.
(2) Another technique, for the early stamps that have the chrysanthemum crest, is to count the number of petals. A genuine stamp will always have 16 petals. If it has more or less, then you can rule it out as a forgery. If it has 16 petals, you can't necessarily rule it in as genuine, however, so be aware. But again, this is another easy technique to be able to rule some out.
And just to illustrate the challenges for authenticating Japan's earliest stamps (the dragon stamps), the expertiser I know always literally plates the stamp, in order to be able to prove genuineness.
For members of the ISJP, they have quite affordable identification services (although actual issuance of a certificate is a little more costly), and the prices are really not too bad for non-members, compared to a lot of other organizations and certification services.
In summary, it is quite a challenge. But mastering those 2 techniques above will really help.