There have been various threads on Natural Mamas, The Babywearer etc about fake ergos. There is a checklist of some things to look for here:
blog.babesinarms.com.au/category/fake-ergobaby-bounty-reported-sites/
There is also more detail on Natural Mamas here www.naturalmamas.co.uk/forum/showthread.php?t=127891&highlight=fake+ergos
Unfortunately some fakes are more convincing than others, and Ergo have made subtle tweaks to their design over the years too so there is some variety within real ergos.
And of course there's the old adage - if it sounds too good to be true, it probably is (such as the 'brand new' ergos on ebay at around half normal retail price. Although it seems some sellers have got wise to this and started selling fakes as 'used' instead....