Recently, I bought a LEGO part from Bricklink (Technic, Steering Arm with 4 Ball Joints with Black Wheel Hub (x872 / x873)).
I'm not sure if it is an original part or not, because I don't see any LEGO trademark or ID number on the element and I cannot find any reference for that part. The only text on the brick is a single digit (e.g. "7").
Quality of the item is really good, so I believe it's original, but I need to be sure.