Believe it or not, most airlines will tell you if you ask the right question. If you ask: "How many seats are available and how many seats are authorized on flight 123?", the agent will most likely give you three numbers:

capacity (total seats on plane in Coach and first)

Available seats: How many physical seats are still available

Authorized seats: How many seats they can still sell

The difference between authorized and available is the number of seats the airline will oversell this particular flight on this particular day. This can be an amazingly large number. My wife used to work for Delta (PS: Never fly them, trust me) so we have some insider information, for example on flights to/from Central America Delta would routinely oversell a 757 or 738 by 60 people and still leave with empty seats.

The other way of guessing availability is looking at the numbers at the tool. IF you see 7s or 9s across, the flight is open. If you see lots of low numbers (2,3,4) and no 9s or 7s at all, it is tight. If it is zeros across it is oversold, which does not necessarily mean you will get bumped. It just means it is really full.

Hope that helps

TIM