The only way to tell is looking at the fare. The higher the fare, relative to much farther out on the same day/flight, the emptier the buckets (levels of discounted tix) .
To see if the flight is sold out, try buying 8 (the max) tickets for your flight. If it lists as available in any fare, there are at least 8 seats. If there are 8 Wanna get away fares, closer in (under 7 days out) you know there are lots more than 8 seats available.
I guess it is more art than science.