Generally not with see and hear (lát and hall). Látom... means "I see..." or "I can see.." Another verb like this is talál - it means "find" or "can find". Nem találom az ajtót sounds better translated to English as "I can't find the door" rather than "I don't find the door".
In textbook-perfect grammar, it should be "Whom can János see?", but many people these days just use "Who" instead of "whom."
It is still clear to us because of the word order. If we wanted to ask a question about if somebody else can see János, we would put János at the end: "Who can see János?"
very very minor gripe here but I'm unable to write "kiket lát jános." the problem, presumably, is either the lack of capitalization or the lack of question mark, and again, it's the pettiest most minor gripe ever, but I think most ppl don't bother w/ caps/punctuation bc I know where to put it when I need it, so randomly getting things wrong is somewhat annoying when in most other sentences neither of these are required, or at least inconsistently so