My 2007 G35 Journey was recently in the body shop after a deer strike (3rd one in 4 years, different vehicles!) and the bumper cover is not lining up properly on the driver side with the hood, fender, or headlight. Also the ICC radar sensor is partially blocked by the cover. So, I'm wondering if they are trying to pull a fast one and using an aftermarket cover when I paid for an OEM. Is there a Certificate of Authenticity that is easily visible from under the car? I pulled the engine cover yesterday and looked everywhere I could see and didn't see anything.

Most molded N/I parts have a part number scribed into the mold somewhere on the back surface, but I have no idea exactly where it would be on a bumper cover. You might need to remove it and inspect the whole back surface. There are some parts with no p/n in the mold, too, so there's no guarantee you'll find anything.