According to the International Table Tennis Federation, a good serve consists of the following:
2.6.1 Service shall start with the ball resting freely on the open palm of the server's stationary free hand.
2.6.2 The server shall then project the ball near vertically upwards, without imparting spin, so that it rises at least 16cm after leaving the palm of the free hand and then falls without touching anything before being struck.
2.6.3 As the ball is falling the server shall strike it so that it touches first his court and then, after passing over or around the net assembly, touches directly the receiver's court; in doubles, the ball shall touch successively the right half court of server and receiver.
2.6.4 From the start of service until it is struck, the ball shall be above the level of the playing surface and behind the server's end line, and it shall not be hidden from the receiver by any part of the body or clothing of the server or his doubles partner.
2.6.5 It is the responsibility of the player to serve so that the umpire or the assistant umpire can see that he complies with the requirements for a good service.
18.104.22.168 If there is no Assistant Umpire, and the umpire is doubtful of the legality of a service he may, on the first occasion in a match, warn the server without awarding a point.
22.214.171.124 If subsequently in the match a service by that player or his doubles partner is of dubious legality, for the same reason or for any other reason, the receiver shall score a point.
126.96.36.199 Whenever there is a clear failure to comply with the requirements for a good service, no warning shall be given and the receiver shall score a point.
2.6.6 Exceptionally, the umpire may relax the requirements for a good service where he is satisfied that compliance is prevented by physical disability.