If you are free to choose which task you work on, you can time the periods that you work on one task. When you are focused you tend to work longer periods on one task.
Just write down what time it is when you start doing something else. If you see a lot of small time blocks getting replaced by larger blocks, you know that your focus increases.
This method may seem to have one drawback: it needs you to register/realize that you are changing tasks, which requires attention, and you need to write down the time, which is a distraction. But hey, realizing that you are changing tasks may be enough to remind you to go back to your previous task!
You can do this by hand or use a computer application. If that is running in your system tray and you can just double click it and enter a new task name/description), it's only a small interruption. I don't know of any one such app (you could ask on Software Recommendations Stack Exchange), I've always used an custom program.
The advantage of also recording a task name, is that you can afterwards distinguish external interruptions from 'just losing focus yourself' - so if necessary, you can do something about those external interruptions.