The AP poll of sports writers has been going since 1935. UPI used to have the Coaches' poll before it was taken over by CNN/USA Today. Many sports almanacs list the "National Championship" based on those two traditional polls.
Of course you can add the polls by various sports magazines, or as I mentioned, some of the "look-back" studies by people such as Jeff Sagarin.
Notice too, that I use "national championship" in quotations, because it is mythical.