Simply see karma in your direct, personal experience (not magic, not "god's will", the world certainly isn't always fair and just). Once you can do that...
Then ask yourself, "With reference to what
does one regard things as internal and external?"
Are things external because they are outside the body and internal because they are inside the body? Are things external because they are outside the mind and internal because they are inside the mind?
On what foundation are things even regarded as internal and external in the first place?
I wouldn't deprive you of the opportunity to discover these things for yourself by telling you to what to believe.
The best things in life aren't things.