theverge.com/23283921/steam-fps-counter-how-to
Please forgive if you already know this...
Open Steam.Go to Steam > Settings > In-Game.
Under In-game FPS counter, select the corner where you’d like the counter to appear. You can also check High contrast color if you prefer.
Click Ok.
Now, go ahead and start your game. You should see the frame rate your game is running at displayed in the corner that you specified.