I just checked my wife's refurbished T3i, it has an almost imperceptible little pin mark by the serial number. Also checked my refurbished 600 RTs, they also have a little pin mark. So tiny, you'd never notice it if you weren't looking.
Another way to know, I think, would be if the owner still has the original box. Refurbished never come with the original box, but always with the "generic white" Canon refurbished box.
Really though, if you are buying used, it doesn't matter much whether the seller bought it refurbished or bought it new. In either case, you don't really know anything about the history of the camera and can only judge by the apparent condition, the price and what the seller is telling you.
Also consider this, even if the camera was once new, you have no way of knowing if you are buying it from the original owner, or if they just bought it used from someone else.
At any rate, if the seller isn't offering the item at less than what you can buy refurbished then they are asking too much, regardless of whether they bought it new or not.
As an aside, I recently saw a new phenomenon on eBay -- people re-selling refurbished items still in the sealed Canon refurbished box. Looks like they bought them during the recent 15% off sale and then when the items were sold out at the Canon store, they were selling them for a slight markup.
