As best we can - which isn't always good.
We ask first. If we hear someone else say it first, we'll try to imitate that person. We'll make guesses based on what we think it might sound like. We may sound it out using whatever letters we're given, keeping in mind rules we half remember from languages we don't really know. The vowels could do anything.
So, I see "Wojtsusik," that looks vaguely eastern European to me - I would guess that "j" probably sounds like "i" or "y," so I would guess "Woy-it-soo-sick."
Also, in English name spellings and name pronunciations are often very, very different (due many reasons including the weirdness of English spelling or weirdness during immigration). So, we're open to correction when someone says "You say it ______."