|It has been suggested that Legal Booster Sets & Promotional Cards be merged into this article. (discuss)|
The reason given was: that should be covered here instead
For the most part, you can look at Legal Booster Sets & Promotional Cards to see what is legal. However, there are a good list of cards that cannot be used.
To start off, there are the original Egyptian Gods. First of all, the back of these cards are not the same color as other official Yu-Gi-Oh! Trading Card Game cards, then on the front of the card, on the lower left hand corner is a note: "This card cannot be used in a Duel" The new Effect Monster versions, however, are legal.
Then there are non-English cards. For the most part, you can use a card that's in a different language if you have an official translation on hand for reference. However, OCG cards cannot be used, even if your have a translation on hand.
In addition, most tournaments use the Advanced Format, so while the Forbidden cards are legal cards, they are not allowed.