When the word now means "at this time" or "currently," it is an adverb and takes no punctuation. When it has no meaning and is just a throwaway, a "filler-type" word, it
takes commas around it.
...I now want to discuss the fight that took place.
...Have you now seen the doctor about this problem?
...Did you say, now, that he was violent that night?
...Well, now, did you see her there?
At the beginning of a sentence, there is no way to tell after the fact. You have to hear it at
the time and get the comma (or not) as it is being said; in other words, it is "You had to be there" punctuation.
Happy punctuating!
Margie