Unfortunately, seeing how many seats are available for sale will not tell you how many have been sold. The problem is overbooking, and each flight allows a different number of seats to be overbooked based on destination, time of day, day of the week, month, past history, etc.

Even looking at a seat chart won't really tell you, since typically 20-30% of the seats on any given flight are blocked for airport-only assignment on the day of departure. Only the computer system that can be accessed by a res agent has the information you want - number of seats sold, number available for sale, number overbooked. If you get a res agent in a particularly good mood, he/she may be willing to tell you want you want to know, though I think most airlines have a policy of not releasing this information. Otherwise, I don't think there is anyway to know for sure. Even the travel agents' computers don't have this information.