[reitanqild]

Practical tip:

If person says да (da) for "yes" I think they speak Russian.

I think Ukrainians use так for "yes". (Someone please confirm or correct me, but I am 90 something percent sure.) (Edit: looked it up and yes, I think I am right: https://www.ukrainianlessons.com/meanings-of-tak/. That said, Russian also have the word так, so try to infer if they say it as yes (e.g. так так так)

Of course many Ukrainians including famously Zelensky himself speak Russian natively even if more and more change to Ukrainian.

So a surer sign might be to say: "Slava Ukraini" and expect back "Heroim Slava" or something to that effect ;-)