How to spot a liar

In 13 after my character kills her daughter she then lies about it to her husband, and to the police. We see her lying and pretending to not know where her daughter is. She is found out eventually, but we don't see how this happens. I have struggled with this scene where I am lying because I can't work out how she would be in that situation. Is she feeling guilt or remorse? Or is she just relishing in the fact that she has nearly gotten away with it? Or is she just petrified of being found out? I researched mothers who kill and made the decision that Sarah must have some sort of psychotic disorder. This means to me that she doesn't feel guilty straight away, she honestly believes she did the right thing. She is only upset because she has lost Dennis. Towards the end of the play when we have the split scene and everyone says a few lines, that's when she is feeling guilt. I see that as when she is in prison probably on medication and realising what she has done. So for a short section of the play she is lying. I decided to do some research into the science of lying. I found this video:

This was particularly useful because of the examples of guilty and innocent mothers. An actual women grieving and someone lying to save themselves. I have considered using this to inform my performance decisions. Maybe I am not as upset as I have been making myself in previous rehearsals, there should be more reservations in my character. The audience should see the performance and my behaviour and know that I'm guilty, even if my character wouldn't. I can't force any of this behaviour and I do not want to fall into a pattern of mimicking the guilty women I have seen videos of, but I want to try and develop my own version of guilt and lying that Sarah would portray as well as linking this to the research I have on mothers who kill and psychotic disorders.