Pre Purchase
Many media manufacturers not only provide supply for the name brands, but also sell under their own label. For example, a quick check at NewEgg revealed that they do carry some spindle cakes of Taiyo Yuden discs.
In terms of the name brands (like Fuji, Memorex, ...), if you can find "made in Japan" somewhere on the spindle cake, then chances are you've got bona fide TY's in your hand. Fuji, for example, is a good bet on finding TY's. But again, make sure you spot the land of the rising sun moniker first before purchasing, else your likely buying cheap Taiwanese media like CMC.
If your not buying via the name brand route, then be very wary of claims that they use TY media. Try to find someone who has already purchased the discs and can verify that its genuine.
Post Purchase
Utilities like Infotool, DVDIdentifier etc., etc., all provide the 411 on your shiny new discs