First, we need to find your save file
• Open file explorer and type %LOCALAPPDATA%\MGSDelta\Saved\SaveGames\ in the address bar
  
• Open folder named with random numbers (it's your Steam ID)
  
• Find UserProfile_0.sav
  
  
• Open https://delta.detta.dev/ in your browser
  
• Drag and drop it on the website

Website will show you every location where you skipped Kerotans or GA-KOs. You can then reference a guide on where exactly within the level they are located.

I made this because after a long run where I tried to find every collectable, I ended up with 63 GA-KO and I was not feeling like doing all of this again to find one duck.

If it helped you, please leave a comment! I'd be happy to hear that I managed to help people 100% this game.