If a player forms any word on the board that an opponent believes is not in the agreed-upon dictionary, the opponent can challenge the play. If the play is valid, the challenger loses his or her turn. If the play is invalid, it must be removed from the board, and the person who played it loses her/his turn. In tournament rules, if player A makes a play, his or her opponent has the right to challenge until player A draws and looks at a tile.
Note: If any challenged word is found to be unacceptable, the entire play is unacceptable.
Note: The challenge penalty is not always universal, and some tournaments (including the World Scrabble Championship, and many North American CSW tournaments) utilize 5-point challenge, where incorrectly challenging a valid play penalizes the challenger 5 points per word challenged (or 5 points are added per word for the challenged player), but no loss of turn. If a player plays a phony which is challenged, the player still loses his/her turn.
These challenge rules suggest strategies:
The playing level of the opponent should be taken into account. If you are playing an expert and she/he plays an obscure 15-point play, it is probably not worth challenging such a low score. However, if you are playing a weaker opponent and he/she plays a word you have never seen before, you may want to consider challenging. You also want to assess the board position if the play is left on the board - if your opponent plays an obviously phony word that gives you a huge play, you should generally accept the play rather than challenge it off immediately.
Bluffing is occasionally implemented in tournaments. However, you should not play a word and try to draw a challenge by saying, "I'm not too sure if it's valid" or "I'm not sure if it's spelled with an A or E" after playing CALENDER. This practice is not allowed in tournament play. Bluffing is most common when the player has a very high-scoring play in which she/he is unsure of the validity, or as a desperation play where any other play leads to defeat.
Additionally, it is sometimes advisable to set up traps for a weaker opponent. For example, you can play THIO, setting up (THIO)L. An unsuspecting opponent who tacks an S onto THIO, making THIOS*, will lose her/his turn.
In club and tournament play, you can "hold" a play if you are deciding on challenging. This does not neutralize the game clock. If you are holding, the opponent cannot draw tiles for 15 seconds. After 15 seconds, the opponent may draw courtesy tiles, but must keep them separate from remaining tiles, and must show them to you if his/her play is challenged off the board. Holding a play can be useful when the decision to challenge a play is more complex.
Lastly, always challenge the final play of the game if you are the slightest bit unsure of its validity.