Hmm, that's a good question. If the character speaking is deliberately hiding something, you'd decide what they wanted you to believe instead, and how that could cause the truth to leak out. But we're unaware of the subtext of what we say, so... Can we use the same technique? Simply choose something that illuminates the subtext by being related to it, or opposite to it? So a woman who refuses to talk about her grief at her husband's death talks instead about she lost her puppy when she was eight, and we know the puppy is really the husband? But if we did that all the time, would it be too obvious?

Building on what Debbie wrote: give your character fear and pain! Your character wants/needs to talk about [X], but addressing [X] is too painful; or, your character wants/needs to talk about [Y], but is terrified another character will laugh at them for it. Pull your characters in two directions: they want/need one thing, but the act of talking about that thing is too frightening or painful to contemplate. But they really need that thing! So the they want to edge up on the topic, talk around it, approach it indirectly, talk about anything BUT that topic...but still get their message across. Put your character in a specific crucible. Now write the dialogue. Voilą subtext. The more fearful and the less self-aware your character is, the more indirectly they'll speak. The braver and more self-aware they are, the more directly they can speak. And where any of us land on these continuums changes from moment to moment.

The simplest tip of all: characters can lie. All the damn time. In fact, why should anyone tell the truth, unless it suits their purposes? If you don't want them to lie, have them evade. Obfuscate. Refuse to accept the premise of the question. Need examples? Just watch a presidential candidate answering reporters in an uncontrolled situation. It's a masterclass in subtext.