Tartuffe does not enter until the third act in Tartuffe. How does Moliere establish his hypocrisy beforehand?
The hypocrisy of Tartuffe is established in many ways and by many different characters before he is actually introduced in person to the audience. This is of course a key strategy of Moliere in order to cement his character as being hypocritical with the audience before he is even presented himself. Note what, for example, Damis says about the presence of Tartuffe in Orgon's household:
Good God! Do you expect me to submit
To the tyranny of that carping hypocrite?
Must we forgo all joys and satisfactions
Because that bigot censures all our actions?
Damis therefore points out what everybody is thinking and feeling very soon on in the play: Tartuffe is a hypocrite and his undue influence with Orgon is making everybody else's lives a nightmare, as Tartuffe is preventing them from enjoying themselves. This impression of Tartuffe as a hypocrite is further supported by Dorine, who, in Act I scene 1 as well, says the following:
You see him as a saint. I'm far less awed;
In fact, I see right through him. He's a fraud.
This quote mirrors the feelings of Damis, but using different words. Dorine is able to "see right through" Tartuffe, recognising that his surface piety is just an act and that he is nothing but a "fraud" underneath. Moliere therefore chooses to establish the hypocrisy of Tartuffe by the reports of others and their account of his hypocrisy. This clearly indicates that the audience is meant to view Tartuffe in this light and be negatively disposed towards him.