Within your Steam library, find the game and right click on it. From there, select "Properties" and then "Installed Files". You should then see 3 options, "Backup Game", "Move Installation" and finally the one you are looking for, "Verify Integrity of game".

Ensure that you are connected to the internet when performing this, as essential data files or the latest files for updates might be detected for the game to run as intended.

This will then scan the files of the game to make sure you are not missing any. The time taken to scan varies from machine to machine as well as the game itself.