OMG It's a first edition LOB set exodia for £3.20!
Well it could be, but it's a bit unlikely. In fact I wrote this guide because a fake Exodia head was selling on eBay and had attracted several bidders, at the time when I was listing one of own LOB Exodia sets for sale.
Telling fake from genuine is not always an easy task. The manufacturers of the real cards aren't completely rigorous. Two 'identical' cards may have slightly different shading, hologram placement, centering etc. Most of the ways of telling if a card is fake have some exception and so you may think your real card is a fake. However there are a few pretty good rules, so here they are:
CONCRETE RULES (ish)
Yu-Gi-Oh cards are the same size. If yours differs from a known geniune, it's a fake. 'Spell' cards were named 'Magic' cards for the first few sets. If you are offered an Ultra Rare Monster Reborn LOB-118 for example with spell card in the top right above the picture, it is a fake. Of course, the LOB set was reprinted later as LOB-EN. It's all about doing your research.
Common cards are plain. Rare cards have silver text. Super rare have plain text but holofoiled pictures. Ultra rare cards have a gold text and holofoiled pictures. Secret rare have a polarizing effect and silvery/multicoloured text. In some angles of dim light, sometimes you can see the diagonal holofoil through the whole card. If the holofoiled picture has some detail, the holofoiling should match it. Cards printed on plain holofoil are probably fake.
Normal monsters with a yellower background have italicised card desriptions. Effect Monsters with orangey background have non-italicised text. The fake Exodia head was clearly part of a fake set where the person had made the four normal monsters first as it had italicised text.
1st edition cards have 1st edition written opposite the card set code just below the picture. They also have a gold holofoil square in the bottom right. Tournament pack cards also have a gold holofoil as do some limited edition cards. Otherwise the cards have a silver holofoil. Because the manufacturers suck, some 1st edition cards have a silver holofoil, but be extremely sceptical of such cards. A non 1st card from a set that has a 1st edition does not have a gold holofoil.
Also check symbols and card text with yugioh-wikia or yugico, which show reliable pictures of real cards. However, if the card has been reprinted, card text may have changed e.g. a LOB set 'Swords of Revealing Light' has different card text to a GLD card. This is because card text was clarified in later versions of many cards. Also check the copyright message for spellings and check the card has a serial number (some don't though e.g. Tri-Horn Dragon and Gate Gaurdian).
The fonts used for each section component of the card are the same accros the range.
LESS CERTAIN FAKE ID
There are other 'rules'. Anything from border width, the feel of the card compared to others when you run your finger along the back, the position of the hologram, the thickness of the font used etc. can be considered ways of identifying fakes, but I find them to be less consistent. The manufacturing was not perfect. These things vary from set to set; print run to print run; card to card. On one set because the brushes were not cleaned perfectly on some packet printers, it was possible to identify which packets held the rarest cards as they had a red dot in a location not on the other boosters.
However if all these things seem to arouse suspiscion then look more closely at the card, it could be a fake.
When running your finger along the back the card shouldn't feel as slippery as new plastic playing cards because of the way Yu-Gi-Oh! cards are made.
Okay, there are other rarities such as ultimate rare and ghost rare, but I don't feel experienced enough to help with those. Plenty of guides help with those. The same goes for 'God Cards'. These are a completely different kettle of fish. I know what to look for for myself, but having never seen one, it would be bad form to tell people what to look for.
Any questions? Feel free to ask, or point out mistakes if you think there are any. I would hate to have misinformed someone.