Could it be just counting returned ballots?
I know in MA early voting is nothing more than an absentee ballot completed at a polling place. If you know the registration of the voter, assumptions can be made by party.
I think it’s safe to say that 90% of early R votes are for Trump. Split the I vote 60:40 for Biden. And 80% Dems to Biden.
Early voters are generally pretty solid. Election Day voters might be swayed...but probably not.
Wisconsin is unusual in that we do not register by party affiliation. We never have to declare what party we vote for, even in primaries. So I don’t know how they are telling how the race is going here. This is part of what makes WI difficult to pole.