Question: "How can I know for sure that my anger is righteous indignation?"
Answer: We can know for sure that our anger or indignation is righteous when it is directed toward what angers God Himself. Righteous anger and indignation are justly expressed when we are confronted with sin. Good examples would be anger toward child abuse, pornography, racism, homosexual activity, abortion, and the like.
The apostle Paul gives clear warning to those who anger God: “Now the works of the flesh are evident: sexual immorality, impurity, sensuality, idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God” (Galatians 5:19–21). Jesus expressed righteous anger over the sins of the people (Mark 3:1–5; Matthew 21:12–13; Luke 19:41–44). But His anger was directed at sinful behaviors and unmistakable injustice.
However, we are also taught to be careful in our anger, that we do not sin. “Be angry and do not sin; do not let the sun go down on your anger, and give no opportunity to the devil” (Ephesians 4:26–27). We should check our attitude as well as our motive before becoming angry with others. Paul gives us some sound advice on the appropriate approach: “Do not take revenge, my friends, but leave room for God's wrath, for it is written: ‘It is mine to avenge; I will repay,’ says the Lord. On the contrary: ‘If your enemy is hungry, feed him; if he is thirsty, give him something to drink. In doing this, you will heap burning coals on his head.’ Do not be overcome by evil, but overcome evil with good” (Romans 12:19–21).
James also gives us good instruction when it comes to righteous indignation: “Know this, my beloved brothers: let every person be quick to hear, slow to speak, slow to anger; for the anger of man does not produce the righteousness of God” (James 1:19–20). The apostle Peter echoes this advice especially for those times when we face those antagonistic toward God and the things of God: “But even if you should suffer for righteousness’ sake, you will be blessed. Have no fear of them, nor be troubled, but in your hearts honor Christ the Lord as holy, always being prepared to make a defense to anyone who asks you for a reason for the hope that is in you; yet do it with gentleness and respect, having a good conscience, so that, when you are slandered, those who revile your good behavior in Christ may be put to shame. For it is better to suffer for doing good, if that should be God’s will, than for doing evil” (1 Peter 3:14–17).
Believers can also channel their anger into constructive action by becoming involved with Christian organizations that combat the influence of evil in society. The key is that, if our outrage results in bringing others into a loving and restorative relationship with God, it’s righteous indignation.