How to check my iPhone battery original or replaced

Also, go to Settings>General>About>Parts and Service History. It will tell you what things have been replaced and if they are genuine or not.

Note that I scrolled down some on the screen to hide the personal information at the top. What I'm showing is in the middle of the screen.

Go to settings - battery - tap battery health and see what it says. An Apple approved battery will show a health level.