This is generally not readily available because airlines don't want their competitors tracking their loads.

Some airlines (SWA for example) have a public non-rev number that will tell you approximate loads (good, fair, bad).

Also you can look on an airlines public websites and try to buy a ticket...this will give you some info, especially for companies that let you select your seat online. The websites will show not available for flights that are within 1.5 - 6 hours from departure so it's not so good for last minute stuff.

The best thing would be to decide which airlines you want to use and ask around to get their non-rev phone numbers.