If you want to see true change in your fitness performance, you should be tracking and keeping note of everything you do. The quote, "only look back to see how far you've come." is perfect. 

If you walk/jog/run, how do you know if you're getting better/faster? Track your runs. If speed or time is your goal, aim to go a little faster each run. For example, if you ran a mile in 10 minutes one day, the next time you run a mile, aim for anything under 10 minutes. 

If distance is your goal, and you run a mile one time, next time go for 1.1 miles or 1.5 miles! 

You can and should be doing the same thing with strength training. Take doing Barbell Back Squats for example, if you can do 8 reps with 95 pounds one day, the next time you do weighted squats your progression should be one of two ways; 1: aim to do 95lbs for 9-10 reps, or 2: aim to do 8 reps with 100lbs. 

Both ways (adding weight or adding reps) are great for tracking your progress.

Doing timed workouts is another great way to check your progress and fitness. Doing a circuit of moves, and seeing how fast you can get them done. Then in a month or 2, repeating that same circuit, and aim to get it done faster.

My own example of this is when I did the 300 rep workout (video above).

The workout is 50 reps each of:
Kettlebell Clean & Press
Leg Raises
& Box Jumps

I first completed this workout in December, with a time of about 25 minutes. I did the exact workout again this week, and completed it in 19 minutes. A 6 minute improvement. That time let me know my fitness has gotten a lot better in a couple months. And I will definitely try it again to try to do even better!

Track everything! The number of workouts, the reps, the weights used. And if you do circuit style training, track your your times. 

Goal is to continue to grow, continue to get better. Every Day.