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 only way to be 100% sure and successful in my experience is do delete the actual folder in Your Drive:/Steam/steamapps/ And delete the GTA5 folder and re-downloading it.
Very recently I did the exact same thing and in result GTA wouldn't let me go online. Verify-ing files frim inside Steam didn't work, nor did Uninstalling the game trough Steam. Only delete + redownload worked.
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.