Generally speaking, card companies mark their cards differently if the cards are actually singed by the players (special card backs, stickers, special card fronts, etc.) that mention the fact that the card you have is an autographed card.
Anything else would be an after-market auto, and unless it comes with some other form of verification, will be questioned by many.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).