Can you tell whether an individual is truly repentant or not?

Joel Beeke explains in this video that when there is repentance, a person's behavior changes as well. There is a forsaking of sin. You can see a person's repentance by the fruit shown in one’s life. When people say “I’m sorry,” it is difficult to tell the sincerity and genuineness of it. While we cannot definitively say whether a person is repentant or not, fruits of repentance help to indicate that the person was genuinely repentant. 

Matthew 7:16-17 (NKJV) – “You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles? 17 Even so, every good tree bears good fruit, but a bad tree bears bad fruit.” 

And the other thing of course here is that when there's repentance in the church, when there's repentance in the nation or in my own life or between people or in the family, then behavior changes as well. There's a forsaking of the sin. You can see the person's repented by the fruits of their lives. So let's say you're a parent and you say, oh, Johnny, you've got to say I'm sorry to my sister to your sister and Johnny says yeah I'm sorry what do you do as a parent you say well you know you sure don't sound like it and you press and you press yes I suppose that's one approach when people say they're sorry however you