Well, I don't follow Yahoo Japan or eBay Japan, but here are a few tips for buying from the English-language version of these sites:
1) If the seller is based in Thailand or Singapore, that's generally a red flag. If they're in Hong Kong, it's more of a pink flag - I've found some good sellers with authentic goods, but I've also seen some horrendous fakes from sellers based in Hong Kong.
2) If there's a Buy It Now price and that price is insanely cheap for the item (e.g. a "New With Tags Prada Dress Shirt" for $50 Buy It Now), that's a red flag.
3) If the seller is using the same exact photos you've seen for the same or similar item from a different seller, that's a red flag.
4) If the seller says "tags have been removed" about something for which there's no good reason why it would be removed, that's a red flag.
Beyond that, your best resource is education. Know the labels you like well - know what lines they have made and when, and know what the labels attached to the clothing should look like. e.g. There was a rash of fake CdG Shirt / Fred Perry bags a while back, but they were easily identifiable because the font spacing of the CdG logo looked completely wrong.
