It is very likely that the candidate who wins Pennsylvania will become president. A problem is that the election officials are not allowed to look at absentee ballots before November 3rd. It is likely that Trump will lead the in person vote and try to declare victory.
However, the party registration of absentee voters is public.
The US Elections Project 2020 General Election Early Voting Statistics webpage is fascinating.
It shows the amazing early voting turnout. One fault is that it has a list of states with detailed data and a map of states showing early votes cast so far as a fraction of total votes in 2016. This currently shows Pennsylvania with a relatively low ratio of early votes = 27.8 % of the 2016 vote.
The page for Pennsylvania shows that (so far) 69.7% of those early votes have been cast by registered Democrats and 20.8% by registered Republicans. This is an extraordinary imbalance. Assuming voters with no party affiliation split equally (which is generous to Trump given polls) this suggests a 49.7% of all early votes lead already booked by Biden. So about 13.3% of the 2016 turnout. Even with very high 2020 turnout that has to be over 10% of 2020 votes.
It will also be possible to do this election night.
I hope that this calculation will make it possible to call the election before the absentee votes are counted. I also hope TV networks do this (no reason not too — the call is always a forecast based on exit polls and models).