You can see seatmaps on Orbitz until roughly six hours before scheduled departure. *Public seat maps like this are not perfect* so you can't 100% rely on them. But they are useful for general information if you understand a few things about them:

(a) Public seatmaps are not useful for airlines that do not automatically assign seats. Most airlines do, but keep that in mind.

(b) Every airline shows certain seats blocked out, and just which seats varies by airline and by plane. These can be good seats (like bulkhead) or bad seats (like no-recline rows).

(c) Exit rows are a particular question mark. Some airlines and seat map sources always show them as full becuase you can't book them. Others show them accurately.

(d) To get an idea of what seats are blocked, check the seatmap for the same plane on the same airline months out on an off-peak day. Right about now, a Wednesday in the middle of January should be pretty light. If (for example) the plane is virtually empty but the back two rows are show unavailable, they are probably empty and blocked out on all seatmaps. Check a few flights and look for a pattern if it is not obvious.

(e) When seat maps appear close to full, the amount of "play" is greatest.
Say you have a plane with 100 seats. Looking at seatmaps on light days, you figure out that they always show the 6 exit row seats as blocked, and the back 6 seats as always blocked as well.

In one instance, you see 60/100 seats as blocked. Most likely the plane is booked to around 48. The back row of seats is likely open because with plenty of of seats available, nobody is likely actually seated there. The six exit row seats appear as occupied but may well be open. Some or all of them *could* be pre-reserved, so you can't completely rely on the flight being booked to 48, but it should be pretty close.

On a different flight, this same aircraft/airline shows fully occupied except for 2 open spots. Is it booked to 98? Could be. But if all 6 exit row seats are unoccupied and none of the 6 bad seats in the back are booked, it could be booked only to 86. On the flip side, this could also be the seatmap of a flight booked to 110 (10 over), some of whom did not get seat assignments at booking because no general-inventory seats were available then. The two empty seats you see on the seatmap now are two people who recently cancelled their trips.

So public seatmaps can give you a pretty good idea of how full a plane is if you're careful in understanding and using them, but there is definitely some swag, especially when a plane approaches full. What I usually suggest to doubters is the next time you actually book a ticket for real and see the actual live seatmap, check out the same flight seatmap from Orbitz. With the exception of certain blocked seats, it usually matches cleanly.