Although not a definitive answer to the question, HaRav Baruch Hirshenfeld, Rosh Kollel in Cleveland Heights, says (taken from a recent Torah Tavlin parsha sheet):
Listening to the Megillah.
Probably the single most important and relevant halacha in the
laws of Purim is related to the Megillah reading. Every Jew -
both man and woman - has the challenging obligation of hearing
every single word of the Megillah. One who does not hear every
word does not fulfill his obligation in Krias HaMegillah.
However, there is a leniency which can make the task markedly
easier. If a person hears the majority of the Megillah read from a
kosher Megillah scroll and fills in the rest of the words (that he
or she might have missed) even from a printed Chumash or
Megillah, one has fulfilled the mitzvah. Although one should
hear it all from a kosher Megillah, it is still valid if it was only a
majority and the the rest is read from a printed sefer. Therefore,
one listening to the Megillah, who has any doubt about whether
he or she heard every word, can say it even by heart (without
looking into any text) and has fulfilled the obligation. This is
very helpful, especially for ladies in the Ezras Nashim (ladies’
section) who might not hear clearly or miss a word here or there.
Important Hint: Some people have a hard time concentrating
on the Megillah and their minds tend to wander and/or dream
away. In fact, they might become so oblivious to the Megillah
laining that they might not hear it at all or they’re in doubt if they
did indeed hear the Megillah laining. The best and most classic
way to remedy this is simply to keep one’s finger on the place
from the beginning of the laining till the end. This helps in two
ways. The physical participation with the reading keeps one’s
mind focused and alert. Also, if at any given time one’s finger is
on the place, he or she must have heard well, and even if their
mind may have wandered, they know they still heard the kriah.
The last paragraph implies that if you do not do the finger following advice, then you cannot know for sure if heard the Megillah when you space out, and therefore you should repeat the words that you may have missed.