obvious signs would be using strange or alternate words in place of the regular words for Attributes or Subtypes.
Look for the serial #, the gold stamp for 1st edition, silver stamp for ulimited edition, and the creator's name at the bottom.
compare the questionable card to a more authenticated card. Check the type font to see that it matches. The closest fakes generally are still longer and thinner than the copyright font that YGO uses.
Look to see if the wording or image appear blurred. Especially the wording. Cursory glances may just think its a slight printing mishap, but its more likely a problem with resolution during the scanning process. That means fake.
Authentic cards use a dull mat gloss finish to coat their cards. It offers less reflection. Many fakes have difficulty reproducing holograms, so they opt for cheaper, shiny final coats to help reproduce the look.
These tend to make the cards thicker and more stiff than real cards, and the colors are actually brighter, more vibrant than the real ones.
Also, because of the layering process that real cards have, your run of the mill fakes tend to put things like the holostamp in the corner on last. This means that it is sometimes easy to pick out, when the stamp comes loose. That doesn't happen to real cards, because they are below the surface.
The back of the cards should have the UDE mark and the YGO trademark symbol.
For real good fakes, check the eye in the holostamp. Authentic cards have a smooth flowing line around the center, where as the fakes tend to be much more square and boxy looking.
Take it from the proud owner of an Ultra Rare Gren Maju de Eiza, Super Rare Lesser Fiend, and English Five-God Dragon.