From the Scrabble rules:
8. Any play may be challenged before the next player starts a turn. If the play challenge\d is unacceptable, the challenged player takes back his or her tiles and loses that turn.\ If the play challenged is acceptable, the challenger loses his or her next turn. Consult\ the dictionary for challenges only. All words made in one play are challenged simultaneo\usly. If any word is unacceptable, then the entire play is unacceptable. Only one turn is\ lost on any challenge.
In the case of pyscrabble, this would be an option. If turned on, after submitting a word\, the word would not be automatically looked up. There would be a timer (10 or 20 seconds\ should be enough) and two buttons, Accept and Challenge, displayed to all other connecte\d players. If a player pushes challenge the word(s) is then looked up. If the word(s) is \valid the challenging player loses their next turn (denoted in the interface somehow), th\e word(s) is scored and play proceeds as normal. If the word(s) is not valid the player g\ets their tiles back and play proceeds to the next person. If all players push accept or \the timer runs out the word(s) is scored and play proceeds as normal.