Signs of this include but are not limited to: over-greased hair, Italian accent (even though they aren't actually from, nor have they ever been to, Italy), frequent references to the mob (even though they know no one in the mob), and using "I'm Italian" as an excuse to be rude and/or fat.
Person: I thought you were on a diet?
Guito: Shuddup, I'm Italian, this is how we eat. Don't disrespect me, I'll get cousin Vinny to break your shins.
Person: You don't even have a cousin Vinny. You're such a guito.
Hey, man, got "guito"? Can you lend me some?