You look for lot/batch numbers near the center hole of the disc.
The numbers should have two letters and six digits like this:
8x DVD+R: TG00xxxx
8x DVD-R: GG00xxxx
16x DVD+R: TH00xxxx
16x DVD-R: GH00xxxx
where “xxxx” are digits.
If you find such codes your discs are genuine Taiyo Yuden.
If the codes are not there (or are different) then your discs are fake.
You may also want to look in these threads:
The Taiyo Yuden FAQ
Brands and disc types that are fake Taiyo Yudens.