You don’t.
It’s not important.
It is also very hard to do.
Let's start with why it is very hard to do. First of all, you need to determine your baseline — normal productivity. This means you need to be able to measure it somehow. How do you measure productivity? Hours of work on something are just hours — time spent, not productivity. Value per hour would be more meaningful but how do you measure value, especially, if it’s an indirect value like the one you can’t sell to somebody. What if it’s something that doesn’t bring value but just reduces waste (per Lean methodology) — like code refactoring, bug fixing, process improvement? According to Lean methodology, this has zero value but you were really productive doing it. Scrum came up with story points to measure uncertainty and then velocity becomes the number of story points burnt per unit of time, but it’s not productivity — you can have a low number of story points but lots of work that is chores and routine.
But ok, let’s imagine, you came up with an amazing fair unit of productivity and somehow you managed to measure it repeatedly and accurately day after day, and got it all logged. So now you can calculate your average, mean productivity and its standard deviation, and you can apply the six sigma approach to it, thus, you can categorize your productivity as average, a little below/above average, significantly below/above average and incredibly low/high productivity that happens once in a lifetime.
So what are you going to do with it? Feel happy when you were more productive and sad when you were less productive? But that’s normal to be more or less productive, that’s why there are a standard deviation and all sorts of statistical knowledge. We are people, we have good days, bad days, flu, sometimes we crush it for no reason, sometimes we can’t get up from the bed.
What’s the value of a knowledge that you can’t use and that can’t change your behaviour, insights or anything else? It just becomes one more thing to worry about, that will distract you from your goal. It’s like logging your calories consumption — incredibly hard to do.
With that being said, it doesn’t mean you don’t need to reflect on things. I believe, sustainability and consistency are the key factors, so in my opinion, you need to reflect on whether you are consistent and whether you can maintain your productivity as a lifestyle. It’s like a diet — the one that works is that one that can be maintained as a lifestyle.
A day, even a month, of incredible productivity doesn’t make the difference. What makes the difference is lifestyle and mindset that allows you to consistently be moving towards your goal while maintaining your physical and mental health.