In several stories about early voting, I’ve seen mentions that Democrats are voting more heavily than Republicans, which is likely something in Hillary’s favour.
How do they know? Isn’t it a secret ballot? 
In many states voters are listed by party registration. You register as a Republican, as a Democrat, as a member of the Socialist Worker Party/the Lizard People Party…or as a member of no party at all. Your stated party preference is at least semi-public, and can be reported as an aggregate. So in PA, where voters do choose a party to enroll with, it’s not difficult to count up the number of absentee votes from self described Dems vs self described Reps.
That doesn’t mean that all the early votes cast by registered Democrats are all votes for Cointon, of course. But it provides a peek into what might be happening.
Another way is… well, to ask them after voting. Polls can be made and that also gives us a clue of what is likely to happen.