How is that possible to know? Exit polling?
Exit polling (I think) is only on election day. I believe it's based on the fact they know who (what party) is voting. Not the actual vote cast, but if a Repub is voting....pretty good bet it's for Trump.
First Early Voting Data Shows Encouraging Signs for Republicans
Some early voting numbers out from Virginia could be an early indication of a strong Republican turnout this year.
www.westernjournal.com