This is made clear in Act I, Scene I of King Lear. Kent observes Lear's interactions with his daughters, noticing how Lear favors Goneril's flattery over Regan's more restrained affections.
Here's the relevant excerpt:
> Kent: "I cannot think...
> That the King loves [Regan] best...
> I would fain see it further proven." (Act 1, Scene 1)
Kent's suspicion is later confirmed by the way Lear reacts to Goneril's initial kindness and Regan's cool response. This leads to his later outburst against Regan, which further solidifies the impression that Lear prefers Goneril.