How do I make line calls when there's no line umpire?

You call every ball landing on, or aimed at, your side of the net (The Code, section 5).

• Any part of the ball touching the line is good. Don't call a ball out unless you clearly see space between where it landed and the line (Code 7).
• A ball you can't call out with certainty is good. You can't claim a let because you didn't see it (Code 8).
• Make calls promptly, audibly or visibly: before your return goes out of play or before your opponent has a chance to play it (Code 15, 17).
• If you call a ball out and then realize it was good, the point goes to your opponent (Code 12).
• In doubles, if one partner calls it out and the other saw it good, it's good (Code 14).
• Spectators, parents and coaches never make calls, and you must not ask them for help (Code 16).
• Opponents get the benefit of any doubt (Code 6).

On clay, if any part of the mark touches the line, it's good. You don't have to show the mark, and your opponent can't cross the net to inspect it (Code 21).