[*]This all said, note that if you get called to a court and a player can't find a mark...that doesn't mean the player cheated...in fact, it could be the opposite. A cheater doesn't challenge a call at first, then once the player walks away from where the ball landed, the cheater asks for a mark...then when you can't find it for sure, he call the official. This has happened to my kid a few times and the official gave the cheater the point!
This is a great point. My player got overruled twice in a recent clay court match. The opponent protested the calls and asked my player to locate the mark. On both occasions he couldn't find the mark. The official came on the court, she had no idea where the mark was. The opponent pointed to a good mark. The official accepted that as the mark. It wasn't. The official has to locate the correct mark without input from either side to overrule.