You can easily tell Uma is lying when she stutters. A lot. If she doesn't admit to lying, just threaten to touch her stomach. Some reason, she always admits it if you do.
//Fun fact: Uma is very sensitive around her stomach, and she will go into hysterics. Oof.//

You can tell Lilly is lying when she says things like this: 'I-I know I am but what are you?!' So, this immediately gives it away. Ack-

When Mia lies, she looks up at the sky, or the ceiling, not making eye contact with anyone. If she doesn't admit to lying, just ask her if Ashana knows. Because, Ashana knows everything.