We can't know. But we don't have any reason to believe the opposite.

With accurate scientific language we don't say "constants are constant". We say "we had never find any experiment or scenario where the constants are not constant".

It's similar to courts. We say not-guilty instead of innocent. If we suspect a guy killed some other guy but we can't find any clue or evidence that this guy murdered the other we can't accuse him to be the murderer. But we can't say neither that this person is innocent.