Nowhere in the play does Parris state to anyone, "Hey--I know those girls are lying." However, you have to conclude that he knows, deep down, that there is something fishy about their accusations. He was there when it all started, he's there every day in court, and he is there when it all fizzles and dies, and Abby bails on him. So, he's got to know. I think that he might suspect it, but, playing along with them works more in his favor than not. Playing along with them makes him look like the important man of the town, an official of the high court, and he even plays the role of a sort of unofficial judge during all of the proceedings. Playing along allows him to voice, in an official capacity, many of his long-held judgments against townspeople that don't like him. He decries Proctor and Corey when they come to the court, trying to turn the judges against him--Proctor and Corey have never liked Parris, and they have all fought in the past. Helping out with the girls also helps him to boost his reputation as a man of god in the town--here he is, helping root out the witches.
But consider the fact that just before Abby broke down and "confessed" to witchcraft, she had been vehemently denying doing anything other than dancing in the woods. Consider that as soon as the townspeople start to turn against the courts, Abby takes his money and leaves--that is so highly suspicious that Parris is visibly shaken and fears for his own reputation and life. Consider that Mary Warren herself came and confessed that they had all been faking. Parris is quick to discredit her, but you have to wonder if he didn't know she was speaking the truth.
The bottom line is that if he did know, he kept it to himself. He had much more to gain from keeping that quiet, and being a selfish, prideful man, he took advantage of the situation to improve his standing and take care of himself above all others. I hope that those thoughts helped; good luck!