How do I know when my flight is full?

Check the airline’s app These maps will show you the layout of every seat on the plane and whether it’s been booked or not. Delta, for example, allows fliers to log in to its app (or website) and view how full the seat maps for their flights are under the “My Trips” tab. 22-Dec-2020

https://www.cntraveler.com/story/how-to-check-how-full-a-flight-is#:~:text=Check%20the%20airline’s%20app&text=These%20maps%20will%20show%20you,the%20%E2%80%9CMy%20Trips%E2%80%9D%20tab.