How do I restore my GTA files?

I added mods to GTA 5 and didn't backup the game files. Now I can't figure out which files are mods and which are not. Is there any way to restore the game files without a backup?

The Verify Integrity of Game Files tool should fix this.

From the Library section, right-click on GTA V and select Properties from the menu. Select the Local files tab and click the Verify integrity of game files button. Steam will verify the game's files and restore them to the originals.

This may take some time as modified files will be redownloaded, but a lot less than uninstalling and reinstalling the whole game.