How do you know, when someone "really" means it?

BAHALOTECHA How do you know, when someone "really" means it? As the Jewish people are about to celebrate Passover, a group of people complains about missing out. G-d then gives them a second chance. Why is there a chance? and why was this mitzvah the paradigm of a second chance? When someone is in trouble, what sign will tell you they need real help?  How does G-d know we meant it?