I too have been burned by the Chinese 'new' N900s but after getting one fake new from ebay and one from a former very active TMO member who famously took money and ran I have learned how to find a good N900.
I typically look for a N900 which is obviously not new but in good condition, you really have to look for one with at least a little pocket wear and the right accessories to prove you are getting OEM and not one with a replaced plastic case, if you search TMO you can find the fake vs real box pictures to help too on really complete phones. I look for sellers in wealthier western countries and tick only the used box on auction site searches.
I suppose it is not foolproof but it has worked for me at least four times.