Upper Deck Entertainment runs an official site of the trading card game, and
they provide a good "Question and Answer" (Q&A) on this topic.
"Q: How can I tell what are fakes and counterfeits?"
"A: Counterfeits can vary in many ways, but you can usually spot them by
looking carefully in the bottom right hand corner of the card. All official
Konami cards have a small hologram there which shows either the words
'Yu-Gi-Oh!' or the Yu-Gi-Oh! Millennium Eye symbol depending on which angles
you look at it from. Most counterfeits have no hologram. Other counterfeits
try to copy the hologram by using a small square of shiny film, which doesn't
show any words or pictures. If you have a card which you know is genuine, you
can check it against one which you are not sure of. If it is smaller (or
bigger) than the genuine card, or has sharp corners instead of smooth,
rounded corners, it is probably counterfeit."
There is an entry on the website of one Kathryn Cramer that has seven tips on
how to identify Yu-Gi-Oh! counterfeits. Here are two of them.
"Fake Yu-Gi-Oh! cards ..."
1. They are printed on generic holographic paper rather than having the pattern
of the holography match the images.
2. The holographc square present with authentic cards is either printed simply
in black and white or is missing.
You should go to the website and read the other 5+ tips that Kathryn provides.
If you need any clarification, please feel free to ask.
Google search on: "YU-GI-OH" fakes OR fake OR counterfeit
Looking Forward, denco-ga - Google Answers Researcher