We know from the outset that Abigail is lying because she acknowledges her deceit when she creates it, telling the others to go along with her.
At the play's opening, Betty Parris is pretending to be sick because she is worried about the punishment that will be inflicted on her and her friends for being discovered dancing naked in the woods.
Abigail takes control of the situation by deflecting attention to others. She does this, however, only after the audience is clearly informed that Abigail had been drinking blood in the forest and performing rites of "witchcraft" as the town sees it.
To protect the group from punishment for what they have done and what she has done herself, Abigail insists that the group maintain a lie.
...she forcefully insists that the girls stick to the story that they were only dancing and that Tituba and Ruth alone conjured her dead sisters.
In the next scene, Abigail denies the truth that has just been clearly revealed when she tells Proctor that the witchcraft accusations are nonsense. She tells him that the girls were merely dancing in the woods.
This scenario offers clear evidence from the outset that Abigail is lying. Additionally, in the first scene she lies about why she was fired from the Proctor home (her story is shown to be a lie in the second scene). She lies also about Elizabeth Proctor's poppet. This lie is recognized immediately by the audience for what it is - a deception intended to incriminate Elizabeth.