how do we know the universe is STILL expanding?
We don't know. This is a depressing thing that makes me, at least, feel so small. The only thing we might assume, that if they once were accelerating that fast, they probably will do it still (even faster), but of course we can't be sure if anything have happened.

It's like this with all the long distances in the universe. How do we know if Proxima Centauri (4.2 light years away) have died/exploded? We wait, 4.2 years.