You have to install an external addon that shows how much damage you are dealing
Basically just download xDamage Indicator or any other addon that does the same thing.

What it does?

Adds a Dummy entity that you can test your weapons and armor on!
Custom armor is not useable, Unless if they use a custom tag that allows their armor to be used on this dummy.

• Add this tag "xdummdumm:custom_helmet" inside your Custom Helmet item tags.
• Add this tag "xdummdumm:custom_chestplate" inside your Custom Chestplate item tags.
• Add this tag "xdummdumm:custom_leggings" inside your Custom Leggings item tags.
• Add this tag "xdummdumm:custom_boots" inside your Custom Boots item tags.

- Enjoy.