[The_Colonel]

This probably happens on a miniscule scale, given the big Russian diaspora in Europe. There are also practical "problems" - a westerner (even western Slavic speaker) has no chance to distinguish Ukrainian language from Russian (or even Russian speaking Ukrainian).

[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 ;-)

[unsupp0rted]

Also… speaking Russian in public shouldn’t sentence a person to be hated, much less slapped across the face.

Oh you were born into a family that speaks the evil tongue? Off with your head, unless there’s some confusion it’s Ukrainian, in which case you’re safe.