In the US, most tournament matches are not umpired. However, you can request an umpire if there is a problem. So when should you seek help from a tournament official? You should get help when your opponent is getting an unfair advantage from something like his serve.
Get an umpire if…
1. Your opponent is throwing the ball into the racket and getting an unfair advantage
2. Your opponent is hiding the ball and getting an unfair advantage
3. Your opponent is spinning the ball with his fingers and getting an unfair advantage
4. Your opponent is throwing the ball back excessively and getting an unfair advantage
Don’t get an umpire if…
1. Your opponent is tossing the ball 4” instead of 6” (there is no advantage for him)
2. Your opponent is cupping the ball (there is no advantage for him)
3. Your opponent is putting the ball in his fingers instead of his palm (there is no advantage for him)
4. Your opponent is tossing the ball over the table by 1” (there is no advantage for him)
5. Your opponent is dropping the ball under the table by 1” (there is no advantage for him)
You should not complain just to complain… and should not complain to break his momentum. However, if he is truly winning points from his illegal whatever, then you have the right to get help from an official. Also, it is important that you mention something in a gentle tone. Instead of calling your opponent a cheater, you might just mention to him that there is a problem (specify) that he might not be aware of and you feel the need to get an official.
Proverbs 15:1 A soft answer turns away wrath, but grievous words stir up anger.
...Or call an umpire?