How can I determine which games I've earned achievements in?

I have put all of the games I have played that have achievements into a spreadsheet, and counted them to 49. I have also added all achievements together which got me to 957. However, my "Achievement Showcase" shows 972 achievements in "50 games".

What can I do to find the last game?

You can easily find out the games that you have achievements on thanks to the Steam library update that came out a while ago.

1. Go to your Steam Library.
   

2. If you have created collections for the games in your library, you need to temporarily disable them. You can do so by pressing the drop-down menu above the library search bar, and unticking the "Group by Collection" option.
   
   You should see a single game category named "All" after this action.

3. Click on the "All" category. You will be shown all the games you have in your library.
   

4. Click on the "Sort by" tag and select "% of Achievements Complete" option.
   
   The whole library will be sorted to show the games with the percentage of achievements earned, in descending order.

5. Scroll down in your library until you meet the first game with "0% Complete" label on it.
   

6. Now, simply cross-reference the games before that first "0% Complete" game with your spreadsheet to find the missing game that you have earned an achievement on.
   

Fortunately, you don't need to go through all your games to find out which ones have achievements, and which ones don't.

1. Go to your profile page and click on "Edit Profile".
   
   
   
   

2. Under "Featured Showcase", choose "Achievement Showcase" if you haven't done so already, and click on any of the slots for the showcase.
   
   
   
   If you have another showcase currently active, don't worry. Steam does not automatically save your profile. As long as you don't click on "Save", you won't have to reconfigure your current showcase.
   
   

3. Finally, open the dropdown. This will show you all games with achievements. Simply select each game one by one.
   
   
   
   

If you have unlocked achievements for the game, you'll see a list of achievements:

Otherwise, the list will be empty (possibly with an error message, if you've never played the game):