Vůbec has several meanings, which are usually restricted to certain types of sentences.
vůbec ne - not at all
A vůbec,... or "... a vůbec" - a slang way to say you don't like something and you ask about that
accentuation, emphasis <- this is used here
"Kdo vůbec jsi?" is like "Kdo jsi?" but stronger, it shows some disagreement or resentment.
How do you know it? How do you even know it? Jak to víš? Jak to vůbec víš?