It is a good idea to have a group you can survive with during a disaster. Of course, you have to be able to trust everyone in your group. There might come a time when you have to confront someone in your group to find out if he has taken something or done something. You can tell if he is lying or not by gauging his reaction.
People who are lying tend to overact. They cannot believe you would question them. They might yell and lash out at you. They act a lot like a wild animal who is caught in a corner. They will do everything they can to escape the situation.
If someone keeps his cool and calmly denies the accusation, he is probably telling the truth.
Keep this in mind so you can keep your group intact. You need to get rid of anyone who is lying and keep all of the honest people together.