I just got hired for a TA role as an undergrad. Normally, I would refer to a professor as Professor LastName if I'm talking to them in a "professor" capacity, or as Dr. LastName if talking to them in any other capacity. However, given the closer role I will have this semester, and that many grad students (all the other TAs) generally call professors by their first name, I'm unsure of what proper etiquette should be.
I know generally asking is the best solution, but there are very few times where the professor stands on their ego enough to say "you must call me by my title", even if they prefer the title to their first name. The prof in question is particularly prone to this sort of behavior (based on hearsay), hence the reluctance to email.
To clarify some comments, I normally stick to calling any professor by their title. However, every other TA is calling the professor by first name (although they are all part of his group). The entire situation of an undergrad being a TA is unusual enough. I don't have the familiarity with the prof that the other TAs have, but it also feels awkward to be the only person calling the professor by their title.