Question: Is it possible that the universe does have a center after all, but we just cannot see it because it already fell beyond the event horizon of our observable universe?
If not, how do we know this for sure if we cannot observe it?

Answer: 
We cannot know “for sure” that the universe beyond the observable universe is the same as the part of the universe that we can observe. But “for sure” is not a benchmark that science tries to achieve (or, indeed, can achieve).

The principle of [Occam’s razor](https://en.wikipedia.org/wiki/Occam%27s_razor) tells us that if we can think of several possible explanations for observable facts, we should favour the one with the fewest untestable assumptions. In this case, a cosmological model in which the universe expands uniformly in all directions and has no unique centre is better than one that assumes a centre of expansion that is so far away we can never observe it or detect any evidence of it.
The universe [doesn't have a center](https://www.livescience.com/62547-what-is-center-of-universe.html), so this question is not well-posed.

If your question is "can the universe have a center anyway?" then you would be challenging the Big Bang, which is something that (as of 2021) is very much in the fringe.