Quite possibly, you can’t.
If you’re in a situation where you have a high degree of authority, you might well be able to associate cause and effect.
On the other hand…
If you know you’re one of many working towards the same end—working on the same issue in a similar kind of way—you probably can’t tell.
It may not be possible to discern which bit of the change you see is caused by you and which bit by other people.
So you just have to trust that you are having an impact, and keep going.
It might be good to let go of the need to see you are making a difference, even though that’s a rather fundamental human trait. The need probably isn’t helping you because it interrupts your flow. Better just to believe.
Can you tell if you’re making a difference? If so, how?