Only guesses based on data, usually not very good:

Seatmaps - A TERRIBLE way to do it, because you can have people with unassigned seats and have no clue. Also, because some airlines block seats until day of flight.

Availability - This is a better way, because you know how many of what fare bucket an airline is willing to sell... BUT, you can't tell if the airline is overselling or not.

Also, there is a fair amount of churn or flux on day of flight, due to customer decisions, weather, missed connections, and a TON of other issues. If the airlines themselves could predict accurately, there would never be an oversold situation. But they can't. And neither can you. And you are at a HUGE disadvantage over the airlines, since your data is limited and your experience base is limited.

When you fly the same route all the time on the same airline, you can use Availability to make some pretty intelligent guesses, but they are still GUESSES.