See in the Chicago Manual of Style FAQ, which is publicly available:
Chicago Manual of Style
Q. Please help clarify a debate over what I see as a groundless but
persistent carryover from high-school English classes: the
comma-before-too “rule.” The rule goes something like this: When “too”
is used in the sense of “also,” use a comma before and after “too” in
the middle of a sentence and a comma before “too” at the end of a
sentence. I am editing a work of fiction in which the author has
rigidly applied the rule. I have just as rigidly deleted the commas.
My managing editor believes that a comma is needed when “too” refers
to an item in a list and has the sense of “in addition” (e.g., “I like
apples and bananas, too.”), but she would omit the comma when “too”
refers to the subject of the sentence (e.g., “Oh, you like apples and
bananas? I like apples and bananas too.”). My managing editor’s rule
helps make a useful distinction, but I am still wondering whether the
comma is ever grammatically justified.
A. A comma can do some work in making the meaning of a sentence clear,
but to claim two different meanings for I like apples and bananas
too with and without a comma before too puts too much pressure on the
comma. Out of context, neither version would be perfectly clear. To
make the different meanings more apparent, short of additional
context, you’d have to be more explicit:
I too like apples and bananas.
I like not only apples but bananas too.
Use commas with too only when you want to emphasize an abrupt change
He didn’t know at first what hit him, but then, too, he hadn’t ever
walked in a field strewn with garden rakes.
In most other cases, commas with this short adverb are unnecessary (an
exception being sentences that begin with too—in the sense of
also—a construction some writers would avoid as being too awkward).
However, at COCA (CORPUS OF CONTEMPORARY AMERICAN ENGLISH), out of the total of
54602 instances of
37350 have a comma
, TOO .
thus comma is much more prevalent "out there" than recommended by CMOS.
Personally, I'd say that whenever the writer wants to suggest a pause/break in speech before "too," in order to make it more emphatic, he/she should use a comma. In fiction, [that would be the case] esp when dealing with characters that prefer inflecsion in speech, or are more "theatral."