“How can I know for sure that my anger is righteous indignation?”…We can know for sure that our anger or indignation is righteous when it is directed towards what angers God Himself. Righteous anger and indignation is justly expressed when we are confronted with sin. Good examples would be anger towards child abuse, pornography, racism, homosexual activity, abortion, and the like.…The Apostle Paul gives clear warning to those who anger God:

Galatians 5:19-21

"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"

Jesus expressed righteous anger over the sins of the people

Mark 3:1-5…Jesus went into the synagogue again and noticed a man with a deformed hand. Since it was the Sabbath, Jesus' enemies watched him closely. If he healed the man's hand, they planned to accuse him of working on the Sabbath. Jesus said to the man with the deformed hand, …"Come and stand in front of everyone." Then he turned to his critics and asked, "Does the law permit good deeds on the Sabbath, or is it a day for doing evil? Is this a day to save life or to destroy it?" But they wouldn't answer him. He looked around at them angrily and was deeply saddened by their hard hearts. Then he said to the man, "Hold out your hand." So the man held out his hand, and it was restored!

Matthew 21:12-13

Jesus entered the Temple and began to drive out all the people buying and selling animals for sacrifice. He knocked over the tables of the money changers and the chairs of those selling doves. 13 He said to them, "The Scriptures declare, 'My Temple will be called a house of prayer,' but you have turned it into a den of thieves!"(Isa 56:7; Jer 7:11.)…

Luke 19:41-44(Jesus Weeps over Jerusalem)

But as he came closer to Jerusalem and saw the city ahead, he began to weep."How I wish today that you of all people would understand the way to peace. But now it is too late, and peace is hidden from your eyes. Before long your enemies will build ramparts against your walls and encircle you and close in on you from every side.They will crush you into the ground, and your children with you. Your enemies will not leave a single stone in place, because you did not accept your opportunity for salvation."

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.

Ephesians 4:26-27

"Be angry and do not sin; do not let the sun go down on your anger, and give no opportunity to the devil".

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:…Romans 12:19-21

Dear friends, never take revenge. Leave that to the righteous anger of God. For the Scriptures say,"I will take revenge;I will pay them back,"( Deut 32:35) says the Lord. Instead, "If your enemies are hungry, feed them. If they are thirsty, give them something to drink.In doing this, you will heap burning coals of shame on their heads."(Prov 25:21-22) Don't let evil conquer you, but conquer evil by doing good.

James also gives us good instruction when it comes to righteous indignation:

James 1:19-20

Understand this, my dear brothers and sisters: You must all be quick to listen, slow to speak, and slow to get angry. Human anger does not produce the righteousness(Or the justice)God desires.

The Apostle Peter echoes this advice especially for those times when we face those antagonistic towards God and the things of God:

1 Peter 3:14-17

But even if you suffer for doing what is right, God will reward you for it. So don't worry or be afraid of their threats. 15 Instead, you must worship Christ as Lord of your life. And if someone asks about your Christian hope, always be ready to explain it. 16 But do this in a gentle and respectful way.[a] Keep your conscience clear. Then if people speak against you, they will be ashamed when they see what a good life you live because you belong to Christ. 17 Remember, it is better to suffer for doing good, if that is what God wants, than to suffer for doing wrong!

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.