I am a Ph.D. student in mathematics, and I am currently applying for jobs. There are some talks that I will be giving later this month and the next few months. Should I put future, confirmed talks on my CV? If so, how should I denote that it is an upcoming talk?
I think there is, in principle, nothing wrong for you to put these talks on your CV provided (i) they will really happen, and (ii) it is clear that they are in the future -- so put a (future) date next to them.
As a graduate student applying for jobs, you are in the same situation as all of the other graduate students: your CV is still pretty short. It is fair to add facts to it that will happen in the future (though it is, obviously, not ok to embellish by listing talks that you only may give but that may not in fact really happen).