For the next time, I'd suggest always doing a look up on O-Review. See if it matches an existing colorway. Of course sometimes it's going to be an OCP/MPH pair. From there you kinda just have to go off the must subtle of clues that you can work with
Another bottom line I advocate is: if you don't know for sure, don't put yourself through the trouble. Granted, this Radar wasn't a major risk situation. I'm just saying for future reference, I personally think that saving a few bucks just doesn't do enough against making a reliable/confident purchase.
My first Oakley purchase was at a bike expo/swap meet. Kinda went in there blind for a Radarlock that just sounded like too good of a deal to pass up. I'm lucky to find was real after meticulously cross referencing it to shop pairs. I even bought from the same guy years later. But in hindsight it was a stupid approach. No way I could get my money back if it was a fake.
For sure, better to do the research beforehand. I buy a LOT of stuff on eBay and this kind of thing happens very rarely, so I guess it's the price you pay for getting deals the rest of the time. Not sure if you actually come out ahead in the end.