There can be so many “academic” answers for this. But in my opinion it’s much simpler than that. It depends on your current level of course, but you can reflect on certain things like:
Am I recognizing more words when I hear people speak English fluently?
Am I starting to speak in English (even if it’s one sentence or one expression) without having to translate in my head first?
Do I feel more comfortable practicing in front of native English speakers?
Etc, etc….
In my opinion these simple concepts are extremely important and they represent improvement. Everyone improves differently and at different rates. If you reflect on these things and notice some slight changes, then you know your brain is expanding it’s familiarity of this language that used to be very “foreign”