Try seeing what Kayak.com shows. They list almost all airlines except SW and JB. Fast way to compare fairs. Bing is similar and shows a prediction if you should buy or wait.
Another option is to see if two separate one way fairs is lower, even on two different airlines. For my upcoming trip this saved me quite a bit by taking Delta there, and returning on United.
Another option is to see if two separate one way fairs is lower, even on two different airlines. For my upcoming trip this saved me quite a bit by taking Delta there, and returning on United.