Tell - tale signs of a RIF include:
Badd speelink and gramer
Inability to comprehend simple terms like STFU noob.
Making you want to bash your skull in.
Also see flamebait.
However, it has also been used by dumbass InuYasha Haters use to degrade InuYasha fans when they don't agree with the fans. The latter is really just an InuYasha version of "Douchebag".
The term originated from the Adult Swim Message Boards to differentiate annoying InuYasha fans from Civilized InuYasha Fans (or C.I.F.s).
Jimmy the RIH: Do you like InuYasha?
Jim the R.I.F.: Yes.
Jimmy the RIH: Well, I hate it, so you suck, you friggin R.I.F.!